easiest way to do it is to go to Easternbeaver.com and buy one of his kits.
A standard 30 amp relay should run everything you want and are cheap and easy to find. A 40 amp is a bit more but I can't see you needing it. grips shouldn't be more than 5-7 amps and the gps next to nothing.
the hardest part is finding a suitable way (and circuit) to trigger the relay.