Sat nav power supply

[wessie]

I think you'll find that all the metal parts connect to the positive side of the battery/alternator is at 12v higher electrical potential than all the metal parts connected to the negative side.

More importantly is the fact that fresh water isn't conductive....Not to 12v anyway.
Problem from rain is not a short. It's corrosion which can eventually result in a short.
So long as the thingy has chance to dry out and doesn't rot, it won't matter if it gets a bit wet.

just for a laugh, connect a jump lead to the + terminal on the battery and attach the other end to a random, unpainted fastener on the engine block...should be quite interesting unless you do it on an old Jag or similar vintage POS