Wiring and Water don't mix.....I need suggestions please

Alright.....I got this 95 Cavalier off my friend for 400 bucks for a good reason. It drives great for a little city car when it's dry. But the first time it rained, the main wiring harness going from the computer in the right wheel well, which is a ridiculous place for a computer, to the firewall, it just shuts down. The fuel pump you can still hear it when you turn the key, but the engine just rolls over and over and over if the harness is wet. Me and the mechanic at the shop where I'm apprenticing tried to seal the computer thinking it was that but it wasn't, but now I've taken the whole thing apart and before going further IU wanted other peoples advice......any suggestions?