Like 64fleetside said, when the ground is bad things can work wonky. Here's an example.... Last fall on my '71 the taillights and marker lights stopped working correctly. When I hit the brakes, everything lit up. Blinkers didn't work right on the back, turn one on and it would light both up solid, turn on the other and both would blink. The fix? Ran two ground straps from the bed to the frame and it fixed it perfectly and haven't had a problem since.
What I'm getting at is this, don't just assume a bad ground would mean it won't light up. It's simply not the case. Always check the grounds first. Just run a temp ground wire to the sockets to test the theory. What do you have to lose but a little bit of time?
