Fuzz, I'd say mount the relay wherever you want, it's probably a good idea to fuse the line as well. I like the KC light type relays they sell at the parts stores for around $5, if mounting under the hood I wouldn't worry about sealing it off. If you mount it in the back of the truck, you could probably tap into the trailer harness for power.
I'm a few miles down the road from you in the Tech Center, send me an email if you want any help.
Pete
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'97 2500 CC 4X4 5spd Sport
'83 K5 Blazer 350 TBI (ex 6. 2), 39. 5 TSLs, 3" lift, Dana 60/GM 14 blt, Detroits