I have an 03 GMC 3500 dually that the driver's side fender running lights won't light up. The passenger side work fine. Also my reverse light doesn't light up either. Could that be the problem?

FLASHER

2000 GMC Truck Savana 3/4 ton Van 4.3L FI OHV 6cyl

The Flasheris located: Front seating area, driver side, under dash, below steering column, mounted in relay block

Are the bulbs good ? Did you check for B+ voltage at the lamp sockets ? The passenger and the driver side get B+ voltage from the same place . Do you know how to do automotive electrical circuit testing with a DVOM - digital volt ohm meter ? Do you know what a wiring diagram is ? How to use it to find testing points in a electrical circuit ? Here is a web site for free wiring diagrams .
http://www.bbbind.com/free_tsb.html Enter vehicle info. year , make , model and engine size . Under system click on lighting , then under subsystem click on exterior lighting . Click the search button , then on the blue link's . The light's on the fenders are maker lamps ! Now here is were it gets tricky , if you look at the diagram on the right you will see the back up lamps are controlled by the BCM - body control module . The BCM gets a turn on signal from the PCM -engine computer , because the transmission range switch is a input to the PCM . This can be very confusing to someone you doesn't deal with this sort of thing . Hooking up a scan tool that can view this type data would be the easiest way to trouble shoot .

ECM Inputs and Outputs Diagnosis

Electric Testing Techniques You Need to Know

seems you have several different problems. High beam working and no low beams=dimmer switch is bad. Reverse lamp not working=bulb socket bad,wiring broke,bad ground at back
Brake light on drivers side stuck on=brake light switch

Check the fuse but it is possible that a bulb can be out

Check any ground wire you can find. They may have gotten corroded and no longer grounding properly.

also I would check both fuse boxes and make sure there aren't any blown fuses.

I have the same problem except the back of mine works but the bottom doesn't. Passenger side is fine. Let me know what you find out if you could.

Sounds like you have a melted tail light circuit board.

they bolt on through the bed which is a hard one to change for most

Check the ground. Sometimes it will work its way loose, not allowing it to start. If ok, check your fuel pump relay in the fuse panel under the hood on drivers side fender well.

Remove 3 - 4 screws on fender cover close to side marker light. Pull fender cover out a bit for you to use your other hand to reach side marker light. By twisting on one direction and here you go the light out. Replace new bulb and replace everything on reverse order.