Trying to trouble shoot the interior lights on this 83 GMC Vandura 2500. I have a short somewhere so I cannot get all the switches to work. My question is, what does the forth power switch do? Also, where is the fuse for this whole instrument panel located? Its hard to find info on something this old... Thanks!

 

Have some updated info in hopes it will help someone else down the road.

Button power:
Left = Rear cigarette
Middle Left = Rear dome light
Middle Right = Reading lights on ceiling over middle seats
Right = Runs to the power switch by passenger / rolling door, then turns on/off lower entry lights be doors.

I found the short for the 4th button. Some funky wiring was going on above the passenger ceiling. However, these entry / door lights still do not come on when a door is open. I am sure this is caused by the funky wiring, but cannot figure it out. Either way, I am happy I can turn them on with the button!

Also, there is a 30 A fuse between this Aux power control unit and the battery. Mine was located under the hood on the driver side right by the battery.