I saw it once in a Nuts & Volts mag, but I haven't been able to find that issue again. It had a relay, 2 or 3 resistors, a 1000uf capacitor and a single spst pushbutton that toggled the relay. Sorry I can't get any more descriptive than that, but I only saw it once and didn't think to write it down.
Any circuit will do as long as it's toggled by a single momentary switch and can handle 30 amps of current @ 12v.
Here is a relay Flip Flop. If you want to use a SPST push button rather than the SPDT, let me know and I'll alter the design. It will need 3 or 4 extra components.