Alright, I'll hook you up to clear up any confusion. Remember, this is a reference...I take no responsibility for your actions.
The red arrows represent the actual worst case current draw. I also labeled each wire as I understand it from your plans and descriptions. I'm also assuming that you aren't tapping power from a fog light or something inside the cab, so you'll have to run another wire from the battery. Typically you shouldn't do anything smaller than an 18 gauge wire for the relay control. Since this line is low current, a 5A fuse is readily available and sufficient for this line.