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

Be careful what you use. Stuff like lemon juice is acidic, so you have to be able to completely neutralize it. Another "made for purpose" terminal grease is the aniti-ox sold by all electrical supplyhouses--and last I knew, Lowes/ HD. This has long been known for use with aluminum terminals

Frankly, many times if terminals LOOK bad enough that they need cleaned, they are really beyond cleaning. This is because stuff like pitting on the surface DECREASES the contact area....and the current handling cabability of the terminal. Replacement of the terminal is the correct answer.

My first bad experience with the infamous Mopar bulkhead/ ammeter was way WAY back in the early 70's. My own 70RR failed the bulkhead connections AND the ammeter started to become damaged because of the plastic housing on the Ralley (Charger style) cluster. This obviously is before Al Gore invented the internet. I drilled through the connector body and ran larger gauge wire "straight through" the thing.