I'm trying to help a friend who's having an issue with his 2500HD.

He says,

"When I put the truck in gear it dings and says the left rear door is ajar. The interior light doesn’t go out. When I try and lock the doors they will not lock. And then finally when I try and set the alarm it will not set. If it does set it goes off by itself."

Does anyone know what this may be or how to correct the problem?

Thanks in advance!

Not sure, but I know the door trigger that tells when the door is open and turns on the interior lights is built into the door latch, and the wire runs through the sill plate, I think it is blue but I'm not sure from memory.