I installed one of these kits on my Crown Vic to allow the use of brighter bulbs, and it left one of the OEM connectors hanging free, which got really corroded.
Also, from my experience, you MUST use relays that have brass spades. The relays with aluminum spades ones will NOT last. The last time I bought some, it was from NAPA (it was the only local auto parts store that carried them, and they cost twice as much more than the aluminum spade relays. While you're buying them, buy a couple of spares and keep them in the car (you won't know you need them until you actually need them). Again, this is from direct experience.
I would also recommend using dialectic grease on all the contacts to avoid having water-caused problems.
Good point on the grease. I used LPS 3 on everything.