The relays should hold up ok under the hood as long as they stay dry. The wiring on mine was in pretty bad shape so I ran all new for the rear of the truck and grain box marker lights, I was able to keep the original wires from the turn signal switch and tied into them in the cab then brought the brake light wire through the firewall and connected into the relays. If your using the existing wiring then you may as well keep it all easy to get at under the hood. Since my truck had no terminal block for accessories I took the time to add that next to the relays. Mine is a Low Cab Forward and the only fuse is a 3 amp for instrument lights and two circuit breakers for the rest of the lighting - all on the light switch. I fed the new terminal block from the battery with a circuit breaker in line to protect all of my new wiring. My box markers are on a dedicated circuit, heated mirrors on another dedicated circuit, and an extra switch that I may use for back-up lights all with lighted toggle switches to remind me to turn them off.
Man, you need to show a picture of you setup and a diagram. I don't think I would ever do heated mirrors, but I can see doing backup lights down the road. I also like the idea of having circuit breakers.
