1998 F150 dies in the rain
 

My 1998 F150's battery dies when it rains. Has been doing it for years. I have two possible causes, my grandpa used to have a shell on the bed, and the wire from it is sitting there uncovered, and the rear sliding window has a crack letting water leak in. A couple years ago, my grandpa added the wrong antifreeze to the truck, and he said that there was a lot of steam and a hissing noise from the passenger side footwell. I have a feeling this could be causing it too. Would the steam have done something to the computer?

I would start with the fuse boxes.

Yep start with the fuse box , also check your battery cables and all your grounds - Clean all your grounds !!

What should I look for in the fuse boxes specifically? This truck has had electrical problems for as long as I've known. My grandpa got an aftermarket alarm system that screwed up something, and the door locks went out a while ago. A lot to work on for this truck.

First look at any grounds take them off and clean them - or check with a meter , next check any fusible links those are harder to identify , down by starter , fuse box area , they can be anywhere , look in your manual the manual should tell you where they are located and how to check them, test all relays sometimes a week relay can react weird in rain , look for any burnt wires .

Could a rusty computer bracket under the kickpanel be an indicator of something at all, or does that just happen normally. There is a leak in the rear window.

You'll want to get any leaks repaired.

Electric + rainwater doesn't mix.