You can do this with ordinary switches. You will need two single pole double pole switches (SPDT) one at each end of your vehicle and a double pole double throw (DPDT) switch in the middle. You run 12V to the first switch, the two outer poles run two wires to the DPDT switch which swaps these over as per position then the two wires go to the last SPDT switch and the centre contact goes to your relay coil. All you need is a light gauge 3 wire run to the furthest switch.
All 3 switches can turn the device on and off regardless of the position of any of the remaining switches.
If you don't like this complexity, use a latching relay, e.g.:
http://www.ebay.com.au/itm/Durite-Relay-Latching-25-amps-12-volt-Cd1-0-728-02-/360538864184?pt=UK_CarsParts_Vehicles_CarParts_SM& hash=item53f1ca7a38
and wire 3 push buttons to operate the relay. Any one press will change the state of the relay. Early VW's used a similar relay for their headlights, probably until about 1980.


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