I highly doubt much more than 6 amps even using the spare solenoid output. And the wire is pretty short. I'm sure 18 would be fine, but it can't hurt to go alittle bigger. The included wire looks like 18 ga, but I'm not yanking it to check. Buy some wire, double it up if needed so you're at least bigger than 18ga, call it a day.
I would recommend doing something to make a standoff, or your passenger might smash the kickplate into your UTEC, bad things happen, etc, then you'd be a sad panda. You might be able to use a long machine screw (~2") that mates with the hole thread, a nut, and a lock washer. Put a nut, then lock washer, thread the nut down the bolt about 1/2", lock it into the screw hole so the bolt is sticking up the same height as the standoff. TXS can probably sell you replacement parts as well, which would be preferred.