where to put relay...

So I have a question. I'm coming from teh boating world where relays are rarely used. I had planned to run 10 ga wire into the rear of my truck for a few lighting mods and just figured it would be easy to have the power source (blue sea fuse block) close. I see the utility in using a relay to "switch" the power from a trigger source (e.g., reverse lights).

So, say I want to tap my reverse circuit in the 7-pin to switch on aux reverse lights that are being fed power from my rear mounted, fused power source. IS THERE A GOOD PLACE TO MOUNT A RELAY IN THE REAR? Should I just buy a waterproof relay and mount it somewhere back there under the bed?

I'm 99% sure i'm going to mount my fuse block behind the tail lights on the passenger side, but there is not a ton of extra room. What is the standard practice? Sorry if this is a dumb question