What do y'all use to clean the metal connectors that go into the bulkhead on either side of the fw?

There are aerosol electrical contact cleaners out there on the market and any auto or electrical parts place worth a chit will carry at least one type. I've got one here I've used made by CRC and it works pretty good and it's never hurt anything automotive I've used it on including bulkheads, but there are certain types of plastic that it is harmful to, I used it on a mini flashlight battery housing that holds the 3 triple AAA's and it first just froze it, then it just disintegrated into pieces, and I've been leery of it ever since.

You can always take real small round wire brushes to clean and provide a good contact surface just to be on the safe side.