Brake lights, common to every car on the road. 
But all brake lights are not the same. Brake lamps
have come a long way and are far more sophisticated
than just turning on the bulbs at the back end of the car. 
So many manufacturers are now running the brake lights
through BCM's, Integrated Power Modules, bulb
detection systems, or the anti-loc brakes.  Along with
just about every other system in today's cars, 
computers are doing a lot of the work for us. Now
diagnosing a simple inop-brake light problem takes on
a whole new aspect than before. This article will cover
some of the basic testing procedures that I use on a
daily basis to determine the location and how to repair a
brake light wiring problem.

The third brake light (also referred as the high mounted brake light) was federally mandated for all passenger cars manufactured by 1986 model year.  1994 the same mandate was applied to all trucks also. That means... as a technician.. if you're servicing brake lights make sure you don't forget the third brake light is working also.  A lot of manufacturer's have made the 3rd brake light independent of the lower brake lights, but that is not true to all vehicles of the same year from the same manufacturer.
Most bulb check systems use current flow to determine whether or not a bulb is working.  If a bulb is bad or a loose connection is in the circuit the current level is changed from the manufacturer's designed amperage flow.  In some cases even the wrong type or cheaply made bulb can have enough of an effect on the current flow that it will set the warning light on in the dash indicating the bulb is faulty.  Many times I've got a car in the shop with a bulb warning light on only to find out the bulb the customer just changed themselves is the actual problem.

Connections are probably the number one problem I find with the exterior lights.  Usually brought on by cracked lenses, moisture in the housing or poor socket sealing. These poor connections can actually compound even more problems with other connections in the systems.  A perfect example is the IPM unit (Integrated Power Module) on Chrysler products.  The leads are tiny and the internal connections in the module are just strong enough to handle a certain amount of abuse.  Increasing the amperage flow can also heat up those connections and add to a premature failure of the IPM.  With most of these modules there is no external repair only replace, and this also means that some programming will need to be done before the vehicle will restart.  (theft system for example).  Among the obvious problems with brake lights there is always the issue of something that has been added onto the vehicle.  Such as trailer lights, extra brake lights, or some sort of added on gizmo to the rear of the car or truck.   Today's wiring is designed to handle the load requirements for lighting as it originally was equipped from the manufacturer and not any added on systems.  Most trucks these days have separate relays and fuse for trailer wiring which should be used for any additional brake light or exterior light (trailer usage).

Diagnosing brake lights starts with determining which type of brake light system the vehicle has.  A wiring diagram is a good place to start, but walking by the rear of the vehicle will give you some idea of how it is wired in.  If there is amber turn signal lenses as well as red lenses then the turn signals and the brake lights are separated.  After finding the exterior lighting section find the rear tail light wiring and follow it back to the brake switch.  If there is no direct line from bulb to switch then you're more than likely going to be encountering a bulb check module or some other form of computer device that the brake switch is feeding rather than directly to the blubs.
Here's a few examples,
2000 Toyota Sienna -  The brake lights are direct from the switch to the bulbs, and uses separate bulbs for the turn signals.
2000 Toyota Camry – The brake lights start at the switch but go through a light failure module before getting to the brake light bulbs.
2000 Dodge R1500 pickup – The brake lights go through the turn signal switch but the high mount (3rd brake light) is wired directly to the bulb.  (turn signal switch could be faulty but you would still have a single high mount brake light)
2000 BMW 740i -  The brake lights start at the switch but go through the lighting module (located in the right kick panel) before going to the brake light bulbs.
Each of these different types of brake light systems have to be tested in different ways.  Even though there are some similarities the actual testing of each of these systems may take you in entirely different directions. As you can see I used the same year models as a comparison and also to show how many different variations you'll find on even the same manufacturer for the same year model.

Starting with the lens area first is usually a good idea.  Look for those cracked lens', moisture, or faulty connections.  Over my many years of repairing these problems I generally can find the problem in what I call no more than “elbow deep”, meaning, most problems are usually only as far as someone can usually easily reach. I don't know of a lot of amateurs that will tear a whole lot of trim and panels out to add something to their car or truck.  So keep that in mind when you are looking for a cause of the problems. 
Check to make sure the bulbs are installed correctly and that they are the correct ones.  A lot of times I find a 3157 bulb in the sockets but not pushed down in correctly or someone has put the wrong style of bulb in a socket. 
I use a test light to check for voltage and ground, (I know old school).  (I use an adjustable hood prop to hold the brake pedal down for me while I run to the back and check the lights.)  If there is no ground or voltage consult the wiring diagram to determine where either of them come from and check there next.  Generally the problem with brake lights tend to be on either end of the system rather than in the middle of a harness.  Unless there are issues with a previous collision or water damage I hardly ever find a problem in the harness.  Once in a great while you might find a scuffed wire where the driver puts their feet or a pinched wire under a back seat, it does happen, but very very seldom.  Occasionally, I'll find the lighting modules have burned out or a BCM is faulty but it's not as common as the usual“elbow deep” repairs.

One of the more common problems is with the systems that use the separate turn and brake systems.  I'll generally check to see if the customer is using a trailer hitch and if so more than likely they have a trailer module (usually called a splitter) mounted under the vehicle or behind a lens.  These units (available at most parts stores) recombine the brake and turn into one bulb which will allow a standard trailer wiring to be used.  After a few years the constant brake light and turn signal usage can overheat the unit and eventually cause a fuse to blow or in some way disrupt the signal path from switch to bulb. 
When it comes to the BCM or lighting module systems that control the brake lights I find the best thing to do is make sure the proper signals are going into the module and check the signal coming out.  This is where a good wiring diagram that is clearly marked is of the greatest importance.  Bad information or poorly labeled wiring diagrams can lead to a false test results. 
On some models (if applicable) you can initiate a signal with a scanner to test the exterior lights and you can also observe the brake on/off signal on the screen.  It's not a complete answer but might save you from doing the old head stand under the dash.  I know I'll take advantage of that every time I can.  (I don't know about you but I've found that bi-focals don't work really well when you're upside down under a dash).
Another common problem is the disappearing brake lights.  Your customer tells you when they put their foot on the brake the park lights go out, or they see the dash lights come on with the brake pedal depressed. This is nothing more than a feedback from the brake light signal and the park light bulb filament.  The most likely cause of the problem is a faulty ground lead either at the bulb sockets or to the rear light fixtures.  These bad grounds can also change the current draw and resistance level that the bulb failure modules use to determine if a bulb is bad.  In some cases even the bulb failure could be illuminated on in the dash. Once in awhile somehow-someway somebody jams the wrong type of bulb in a socket and the same thing can happen. 

Keep in mind the brake light switch has a lot more functions than just the brake lights.  Everything from controlling the cruise control, deactivating the TCC solenoid (Torque converter lockup), releasing the emergency brake, and operating the shift interlock system.  So when it comes to diagnosing the brake lights... take a little break and study the schematics, examine which type of brake light wiring you're looking at, and you should be good to go.  Then you can put a stop to those pesky inop brake lights.