Need Help- Truck Won't Start!

[Shaun1162]

So after my last thread (http://67-72chevytrucks.com/vboard/s....php?p=5728562) I thought my electrical problems were solved... Guess I jumped the gun on that one. I've searched alot on this site and the rest of the net, but haven't found anything quite like this...

So after a new battery, new alternator, and a tune-up the truck ran fine. Drove it around the yard quite a bit. I had problems with the carb and decided I should rebuild it... So before taking it off I went to start the truck and move it out of the way. Had to dump gas down to get it run, but on the last time trying to crank it over it just died (not like a battery dying- just wouldn't crank). Figured it was something simple like a battery cable, so I just left it there, took the carb off and rebuilt it.

Today, I come back, put the carb on, and nothing. The dome light in the cab works, and when I turn the key the dash lights come on (Choke, Brakes, and Seat belts, along with the buzzer for the belts), but as soon as I turn the key to crank the engine it all dies. The first couple times I heard a faint clicking from the starting, but nothing at all now. When I try to crank it all the lights go out, including the dome light. If I close the door, let it set for a second, open the door and try cranking it again- the same thing happens. I used a test light on the fuse block and everything lights up, until I try to crank it and then all power goes dead.

Headlights also don't work at all. They won't turn on with key on or off. The dome light, windshield wipers, and radio work with the key on, but again go dead when I crank it.

I've cleaned the battery to engine ground on both sides, as well as the ground wires going to the radiator support and the frame next to the engine.

With a test light, I have power at the starter, and I've tried to jump the starter with a screwdrive on the solenoid, and it sounded like the starter wanted to engage the first couple times I did it, but now it does nothing.

I've tried all of this with just the truck's battery, as well as jumping it from a running truck..... I could see a bad starter being part of the problem, but why won't the head lights come on, and why do I lose all power after trying to crank it? Maybe it's in the ignition switch?

Any advice is greatly appreciated! These kinds of problems get kind of fustrating...

Forgot to add- Truck is a 78 GMC with a 6 cyl 250 and a SM465 four speed.