This is the sort of readily available and cheap relay you want:

http://www.vehicle-wiring-products.eu/product.php/1216/12v-40a-relay

Just cut your current +12V supply to the nav and connect either end to the 30 and 87 terminal (the load). Use the spare connector behind the dash panel...



...to provide a +12V ignition switched supply to terminal 86 and use its earth to connect to the 85 terminal to complete the relays trigger/switching circuit. Stash the relay inside one of the two panels behind the front indicators.

Make sure the load has a suitably rated inline fuse fitted.

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