Just edited my post above but yeah, just find a wire that's only hot when the key is turned and tap into it with the thin (3amp) wire.

For what its worth - the "weatherproof" version is simply that rubber cover you see in the pic and a resin fill on the back of the block (that subsuquently cracked when I installed it) covering up the connections.

Some pics of mine here.

If you do get that unit make sure you protect the leads on that breaker as you see in the last pic (I used some of that paint-on rubber stuff). Before I did that I touched it with a wrench that was touching a grounded part of the truck and there was quite a spark.