gkid has the same info that I have in my diagnostic manual for 2003 vehicles (it's 10" thick!).
What I would do is check each wire on the relevant circuits for:
1. open continuity (using an ohm meter)
2. short to ground
3. short to power
4. high resistance (using a load test, which you can make for yourself by stringing a few 12 volt parking lights together and putting that into series w/the wires you're testing.
5. check power & grounds
If none of that works, and if all your connectors are clean and making a good connection, and if all your components are in good condition and working as expected, then you may have a control module issue.
This is a PITA to track down, but it's what you have to do.
I just did that on a neighbor's GM Silverado that had no brake lights.
First thing I checked was the fuses, but I MIS-READ the diagram and didn't notice a MISSING FUSE in the underhood fuse box...duh!
After an hour of testing all the wires and circuits and cleaning the grounds, I referred back to the wiring diagram and fuse box layout and discovered that I mis-read the fuse box layout!
So I got a fuse I had lying around and plugged it into the spot and it worked.
Before that, I first changed the brake light switch b/c it was not reading when it should have.
So the only thing I could figure is the brake light switch was stuck ON (a known problem w/this truck).
And in order to save the battery, the previous owner pulled the brake light fuse instead of changing the brake light switch!
Anyway, even if I put a new fuse in it at the beginning, I will would've had to do the diagnostic to test the circuit.
B/c more than likely the brake light would have been stuck ON.
Just goes to show that no matter what you do, you STILL have to go through the process of elimination to narrow down the possibilities.
Next week another neighbor has a VW Jetta that has a right side rear wheel speed sensor not reading that needs fixing.
Turns out that's a common problem w/them.
So I absolutely MUST check the wires to eliminate the possibility of it being the ABS unit.
The ABS units on these are known to go bad.
No matter what, you gotta take the time an aggravation to do the tests on the wires, connectors and circuits if it isn't an 'easy fix'.
