My Blazer won't start. As I start to turn the key, the dash lights up, everything seems fine, but turning key to start, shuts everything off. No starter clicking, no sound, no lights at all. What I do hear is a clicking sound up under the glove box until I turn the key off, and take it out of the ignition.

I started it up like normal yesterday, and drove about a mile and half down the road on my way to work. Stopped to get gas, and when I tried to start it to go, I got nothing. I carry a jump starter with me, (to help other people with), and tried that and it made no difference. So I called a tow. When he got there, he had to put the truck in neutral to slide it up his flatbed. We got it home, and I figured I'd try it again, and see what happened after tapping the solenoid a few times. It fired up, but the battery would die if I wasn't applying the gas.

I started with the battery. It's registering 12.4v. The cables all appear in good condition, and the connections are clean. (I replaced the positive bolt because the old one was stripped). Still get the same 12.4 reading.

So, in accessory, I get every light (and radio) on. Turn the key to start, and everything immediately goes black, except for that clicking noise under the glove box.