GM oval relays are huge (and probably old tech or something, but they work) and are water resistant, if not water proof.
There are a ton of water proof connectors out there, I just stick with the Delphi stuff because it's what I know. Micropack is more compact than weatherpack, and is available in connectors with more terminals. Depends on how much current and wires you have, as to what will work best. Delphi lists it in the various catalogs.
Wrecking yards are ok as a source, but if you want clean wiring, you'll need to get into making all the connections yourself (crimping, seals, terminals). Used connectors are normally ok, I just don't like joining wires together where they have been cut. Prefer solid runs if possible. Just depends how you like to do your wiring.
I *do* cut my wiring out of wrecking yard rigs though (it's basically free around here, if I buy something else), and the GM FWD junk from the 1990's or 2000's (I really don't care about the cars, so I don't know what vintage they are) has a huge run of wiring down the drivers side of the car, right against the door sill on the interior. If the seats are out, you can just rip the trim/carpet out of the way to get to it, and it's a good 6ft of wire, and on at least some of the vehicles, if you spend the time to work it loose, some of it runs all the way to the tail lights, probably 8ft or so.
