The bulbs don't need to be blown to be the problem. I can't tell you the physics of it, but I've had it be the case. It's the cheapest place to start eliminating a new electrical problem like that too. (NYT you obviously know you've got NE salt problems). As far as I know the ground is one of the prongs in the socket, and just based on the mess of grounds I've found in the cab, I'd be willing to bet that it's actually one of those, but I haven't followed it myself to tell you for a fact. It would suck if it was one of the cab grounds, because the short could then be anywhere along the length of it.