You need better humidity control in the vehicle. While they are apart, after cleaned I would tin the contacts with silver solder, if I couldn't find suitable replacements, and put some dielectric grease in it.
Brand may not mean much. If this is not a somewhat standard switch, it might have been made custom for the amp manufacturer. If it is somewhat standard, go to an electronics supplier like digikey and use their parametric search for switches with same # contacts, mechanical mechanism, size, etc.
Since you'd have to take the switch off the board anyway, you should have taken it off and then provided pics.
Another option is to map out the switch positions and pins going open and closed circuit with each position, then put panel mount switches on the amp casing to use instead. I'd strongly consider that if after all the work you'd have to do, then only expected 4 years additional service. Another option - presuming you always keep the switches in the same position appropriate for this application, simply leave the switch in place and solder jumper wires between the appropriate contacts so the switch contacts don't need to conduct well. Maybe also put a blob of epoxy on the switch knub so it can no longer be moved.
As long as the switch contacts aren't gold plated (then probably just need hardened grease removed with a solvent), one of the easier ways I've found to clean up corrosion and old hardened grease is to put a cotton swab in a rotary tool (on lowest speed) and brasso metal polish on the swab. Brasso (and some other) metal polishes have the slight abrasive, oxalic (or other) acid, and a petroleum solvent, then I just scrub remaining residue away with a toothbrush and IPA or detergent solution then water rinse.
