Turn signal and brake light not working.

check fuses or the turn signal switch is bad

I had an issue like this one on a 2007 Colorado. I assume you mean the turn signal indicator inside the truck lights up. It was a bad bulb. think it was the turn signal bulb on the side that is lighting up. As for the gear indication being dim there is more than one bulb that lights them up and one may have blown. I hope this helps.

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

check wiring harness behind light assembly for a loose or broken ground wire. this would not affect the third or "cargo" brake light

Check under hood fuse box make sure L. trailer light fuse isnt blown.

The fuse box containing the turn signal fuses is under the hood forward of the brake master cylinder on the drivers side.
The bottom of the fuse cover should show you which one of the fuses is the left turn signal and right turn signal fuse.
Good luck,
Regards,

check fuse's, if that doesn't work check bulbs

the source of this problem is the turn signal switch
STANDARD MOTOR PRODUCTS Part # CBS1148 Price $100
Source: rockauto.com

I just had the same problem on my '04 Silverado. All it was was corrosion in the socket/bulb. Put some dialectic grease on both the bulbs and worked them in and out a couple of times and now they work fine. Was weird that they both went out suddenly and together but I guess they are made exactly the same.

if you have the circuit board type rear lights the circuits inside will brake and cross eachother that will cause your problem