Hi
The jack should not fill up with water. Best guess is that there are some drain holes lower in the body that have plugged up. They need to be fond and cleared.
Rain water by it's self is pretty non-conductive (at least as far as blowing a
12V fuse). Best guess is that the problems are at least in part from corrosion. The water problem likely has been there for a while and things have been rotting away for weeks / months.
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Grab an air hose and blow everything out. Tear it down as far as you can to get at as much of it as possible. If it's packed with dirt / corrosion, scrub that off. Let any water dry off, either sitting in the sun or indoors. Spray it down with a lot of WD-40 and let that run off for a bit. Repeat the WD-40 process a couple of times.
While you have it torn apart, look for things like wires that are broken or insulation that has rubbed off of wires ....
Lots of fun.
Bob