Lower brake lights dont work,third light does

Most of the time the problem is in the turn signal switch. The third brake light circuit bypasses the turn signals.
Someone would need to check brake light power in and out of the turn signal switch.

The lower brakes lights are not on the same circuit as the upper third "high" mount, so this is either a fuse issue from a short in the system or it is a brake light switch problem, if the park lock works (auto transmission only) the system has power and the problem lies in the wiring. attached is a link to a wiring diagram source, trace the circuit. http://bbbind.com/free_tsb.html

check the light socket it could be lose on the wires or just the light

some fault code stored in your car, if you dont go to garage repair it,
i suggestion you get a code scanner to clear it.
maxiscan us703 can do your car.

The outer brake lights work off the brake light switch mounted by the brake pedal so that will need to be replaced. The 3rd brake lamp is powered by another source with a power inverter that runs up the voltage somewhat for the lights are light emitting diodes instead

I've worked on these the first drove me crazy. its the turn signal switch every thing runs thru these. head lights, turn signal, brakes. except the third brake light. it runs directly from the brake switch. go figuare

A very common issue with these is the multfunction/turn signal switch in the steering column.GM has a recall on this problem you should contact your local dealership have your VIN number to see if your vehicle is included in the campaign. If so the repair should be minimal cost or no charge to you at all.

The brake light switch on your truck, is wired so the brake lights and 3rd light are on seperate circuits. Check the wiring right at the connector on the switch. Pull on each wire individually and you will find one that is being held together by the plastic insulation only.

does the 4 ways work? if they do it would be the brake light switch at the brake peddle,,,,,,,,, if they dont the problem is in that red hazerd switch on your steering colum inside it has 3 spring loaded pads that make 3 electrical contacts each so 9 in all and they get sticky from dirt, dust & coffee and hot chocolite dumped on them and stick and burn out. and your brake lights stop working... I could tell you how to fix it but rather diagnose what it is first

ljones, Did you check the "VEH stop" fuse in the left side instrument panel fuse box. There are 3 stop light fuses, 1 for all the stop lights located in the under hood fuse box. 2 in the left side IP fuse box. 1 for the upper stop light and 1 for the lower stop lights. The fuse box in the IP is where the upper and lower stop lights split. Let me know.