You'll need to tap into the brake light switch, 12v power, and have a good ground (I think that is all but it's been a while). All the brake controllers I've used had decent wiring instructions, but you'll need to supply the wiring from the cab to the plug, circuit breaker, etc.
EDIT: Depending on the trailer, you may need a constant 12v lead to charge the battery for the breakaway system. I think my trailer uses the same circuit used for charging the battery for camping trailers, but I have a 7-way plug set up for that use.
I decided the past few times that it was easier to just have U-Haul install the controller and do the wiring....