I put it into gear and it dies.

Or you could buy a new middle connector and some new terminals.
Depends on how serviceable you want to make it and well sealled.
DMT makes sells the seals.
Either way, I'd pop all the metal terminals out and clean them.

Maybe you've seen these, but if you haven't.
A bit of Wiring 101

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

There's more threads you can look for including a few on splices. All pretty good IIRC.