03 Silverado - 3rd Brake Stuck on, no Dome Light
 

My name is Randy and I am Uber Man on YouTube. I buy wrecks to rebuild for my channel. I usually have no issue diagnosing electrical but this one has me stumped and I just know these issues are related. I have a 2003 Silverado 2WD, 4.3L, Reg Cab, SWB w/ factory tow package. I have four issues that I believe are related as follows:

The 3rd (CHML) brake light is stuck on unless the truck is not running. If the truck is running the 3rd brake light will remain on. The normal brake lights function properly. When I pulled the CHML fuse, the 3rd brake light remained on. When I pulled IGN 3 fuse, the 3rd light shut off. When I unplug the BROWN plug from the I/P fuse block, the 3rd brake light goes off.
The cargo light does not function at all. When I push the button for the cargo light a yellow dash light comes on showing the light is on. Both bulbs are good. Wiring appears to be fine as well.
The dome light will not work unless I directly wire a ground to it (from the blue wire to the frame. It appears the black wire is 12v+) With a ground connected the dome light comes on BUT it will not turn off. Even with the doors closed, the dome override switch pressed, it remains on unless I disconnect my test wire.
The rear view mirror has an airbag on/off light. This light does not work at all. The light on the override switch does work properly.

I feel that all of these issues are probably related to the brown plug on the I/P fuse box. Maybe they are not related at all. This is beyond my capabilities. There are no codes, and everything on the truck functions properly except these aforementioned items. This truck has no options other than A/C and cruise control.

I have pulled the headliner, pulled the A pillar trim on the driver side, torn the dash apart, pulled the sill covers on the driver side. I can not find any damage to wiring anywhere.

It may be worth mentioning that there was an aftermarket stereo / amp / subs installed at some point. (It was back to factory when I got it but the old wiring was left behind and not done well) I have not removed the radio but the factory stereo always says "CAL ERROR" and will not remember stations, or settings other than time.

Please, help.

Thank you,

Randy Shear

Hi Randy, I have been reviewing your problems listed in your post. I also have watched your video on youtube where you talk about your issues with this truck. I have a few questions that may clear things up for you. I should explain that I am a GM dealer technician and decided to chime in and help out because your issue peeked me interest. I'm an electrical aficionado and love to tackle problems like this.

My first question is when you say you unplugged the "brown connector" - is that on the passenger or driver side IP fuse panel? Both of these fuel panels contain brown connectors and it will make a difference as to where we go from here.
Second: Do the HVAC functions work okay? All blower motor speeds, actuators for defrost/vent/floor?

Also if you would, please clarify your voltage readings on your cargo lamp socket for me. Your "blue wire" should actually be "grey w/black stripe" which I can see being easily confused. The solid black wire here should read straight ground. Literally read 0.0v on your DVOM. If it reads anything close to battery voltage, your issue lies with voltage backfeeding through your right side IP fuse panel and quite possibly related to ground g200. I cannot help you specifically as to where this ground is, but the description in the service manual says "In the HVAC Harness, on the Right Side of the I/P Near the A-Pillar Below the I/P Top Cover"

The next peculiar thing I see is that you mention the "IGN3" fuse. From what I can tell, this fuse should only be installed in your fuse block if your truck is equipped with RPO code "NYS" - in other words if you have a rear steer truck. Based on your video I do not see this to be the case so perhaps IGN3 fuse should be eliminated at this point to help clear some diagnosis up.