Best not to mount it in the spare wheel well, as they collect moisture, behind the nearside seat upright on the metal work is a 8mm hole that is ideal for mounting a relay holder, added bonus is to take it out at shows etc from inside the boot quick and easy, a 20 amp relay is fine, spst or even changeover can be used, then just some 28/030 wire, I would(and did) run the earth straight from the battery as well, although a lot of people dispense with the inertia switch as well and wire straight of igntion feed to relay.

tabetha