Are all the lights inop, or does the center-mount still work?
Ok... does the center-mount (the one on the cab) still work, or is that out as well?
Ok. If all 3 are out, then most commonly it is in fact, the Brake Light Switch. It's located at the top of the brake pedal arm, under the driver's side dash. Replacing that switch should solve the problem. Always remember to check the fuses as well.
Ok- can you check fuse #1 (labeled "STOP/HAZ") in the instrument panel fuse box?
Correct. The next thing to check is going to be the wiring harness going to the switch. Look for any broken or rubbed through wires. Also make sure the crimps on the wires are tight, and the wires have not "pulled out" of the crimps. I can provide you with an electrical schematic of the switch, it's pretty basic. If the wires look good, i think you may be looking at a problem in the VCM (on-board computer)
Correct. If the brake light switch is bad, the cruise will not function properly either. If you are having problems with cruise, and got the new switch from a "cheap" parts store, like say Auto Zone, there's a strong possibility it's either the wrong switch, or a defect.
No need to go to the Dealer. The guys over at NAPA know their stuff, and have very high quality parts. I never recommend AutoZone for electrical parts.