That makes perfect sense, when I connected it all I left a section of original wiring coming off the back of the dome light and down towards the fuel tank, I didn't want to remove the dome light itself and risk damaging the brittle bits of it. I would imagine that even though its a slim fit a rat must have squeezed up there and chewed a spot in there that's grounding out against the cab.
I will remove the entire dome light and run brand new wires through the whole deal. I hadn't been leaving any original wiring in place if any section of it was damaged except for this and now I know why.
