When you say the battery went to half power, do you mean you went out and it actually measured 6v after only turning the key on? Or to START?
Where did you measure the power? At the battery post, or starter relay end of the positive cable?
Just for giggles, reach down behind the ammeter and find the bullet-connector in the large Black wire (might have a yellow stripe). Pull it apart and inspect for corrosion or looseness or any other fault. Other than the battery cables, this is (usually) the only thing that can kill the power to everything.
If it looks ok, put it back together and make sure it's a good tight connection.
If the cable, the positive cable sounds bad. When I had a bad cable, it would click when I hit START, but would instantly die and so would everything else. No more clicky no more starty.
But if you measured it at the post, I'd say bad battery. I can't think of anything on the truck short of a fully stuck or shorted starter, that could draw a battery down that far that quickly. During a normal start, the battery might read 10v or so, but not 6.
Have you connected another battery to it to see what happens?
Firs thing I'd do is at least turn the engine over by hand to make sure it's not frozen up. I doubt it is, but you never know, and that would certainly put the hurt on a starter.
How old are the cables? Check them for tightness and cleanliness, but if they're more than just a few years old (and even if they aren't) a new set of three 4ga or 2ga battery and starter cables definitely qualifies as cheap insurance.
I don't know what would make a battery boil like you described, but if a cable was shorted you'd see and hopefully smell it from it's melting. Be easy to find that kind of fault in a quick inspection.
Good luck. Probably a battery. Not the end of the world, and I can't see how anything to do with the turn signals or any of the related circuits could do this. If so, half of your wiring harness would have melted with it.
Paul
