pull off the panelling and follow the wires to where it's screwed to the body. Ground is black

 

Thank You again, Joe. Will start looking when time permits and will report back.

 

Here is a domelight schematics for your 2001 Express... if it could help

 

Thank You for the schematic Pomerlo. I have been in the weeds and have not had a chance to dig into this. I will report back with results.

 

UPDATE: First, Thanks to all who helped with this problem. To recap, my cab courtesy light and the lights in the box (Cutaway van) were not working. At the drivers feet, under the rubber mat is a bundle of wires under a Velcro cover and wrapped in tape. Dome and tail lights run through this area. The lighting circuit had a spot where the wire split to two wires (one to dome, one to box) This connection looked like it was poorly soldered and was very corroded. The same went for the ground wire. Cleaned the whole thing up and all is working well. I hope this helps someone in the future,