I would use one of these:-
http://www.ebay.co.uk/itm/Latching-r...tem1c4fb1 d89c
Use a push button switch with a tell tale LED in it. The switch can be mounted wherever suits you, but you need to run a four core cable to the unit. Two cores for the switch operation and two for the LED. These only carry a very small signal current so the cable can be thin.
You could supply the unit from an ignition switched circuit.
Wired this way means that when ignition is switched on the relay stays in the OFF position. One push turns the relay ON, another push turns the relay OFF. If the relay is ON and you turn the ignition off the relay will also turn OFF, and will stay OFF when the ignition is turned on again.
The tell tale LED in the push button lets you know what state the relay is in.
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Ray at Wigan Pier
2014 A8L D4 4.0TFSI Gold (Sticker says "Beige") Can't leave you in a black smoke cloud anymore..
In progress, Nothing left to do.
Gone 2004 A8 D3 3.0 TDI. Ebony Pearl Black (with little bits of other colours and glitter)