You're a bit unlucky if it is within the amp itself and not something less fiddly and external. Opening it up and having look you should be able to see what the state of it is. Amps don't like water as a rule...
It may not be a solder / corrosion issue have a look for any bulging / corroded / leaking capacitors too in the affected area as a damaged one of those (either directly by water or having electricity through it when wet) could be shorting the signal away or going open circuit. Depending on the amplifier design, some sort of capacitor may well be the first thing the signal sees on the way into the amp for electrical de-coupling on the input stage.
I still reckon you're looking as the signal input side, rather than any other part of the amp.
If you have the amp open, remember that although it's fed from a 12V supply, you may well have higher voltages inside the amp - 12V isn't enough voltage for decent volume, so if you change any cappys then observe capacitance and voltage ratings, don't assume 12V will do. I've seen this done.........
Good luck with it, let us know how you get on