One could tap the harness, but when the fuse box has easy to use plug and play ports, why tap a harness? Over time, harness that have been "tapped" tend to corrode inside the wire and can lead to problems down the road. (trust me, a "poke" from a test light alone at the coil harness took my Jeep down all last summer!)
I agree with using the relay to switch a heavy gauge fused wire. Just do it from that fuse box shown above, and run it to a cheap fuse box found at any auto part store. Problem solved, and problems down the road solved too!
Just for kicks, and because I am a nice guy... This is a photo of the box out of my 95. (Gotta love the AMC logo still on the top of the box, huh)
*The three along the left side are all 12v switched in run/accessory.
*The one to the right of the 15amp fuse labeled "ACC" did have 12v in run/accessory, but seemed to turn off after a couple minutes with the key on....
*The one listed as "Clock" is 12v battery and is not switched, but solid 12v.
*The one to the right of the 15amp fuse listed "ING/LPS" s kind of cool, it has 12V switched, but in run only. This would be very sweet to use for the power side of a relay for driving lights. You could toggle switch the ground side of the relay switch and your driving lights would never turn on if the key was in the Accessory position, and turn off with the key.
All of these ports will take a standard male spade terminal, or better yet scrounge the junk yard for that old style radio plug shown in my first photo. I had my radio power wires hooked to two ports for about 10 years and never had a problem with the wire falling out. Just keep in mind, these are not fused ports!