What you're looking for is a combination of a "latching" relay and a "toggle" relay. Fortunately, you can construct one out of a DPDT latching relay by arranging for pulses to reverse its remembered state.
As for the car wiring, 10ga is good for 30 amperes or so.
(no subject)
As for the car wiring, 10ga is good for 30 amperes or so.