The easiest way to get an ignition switched feed to trigger a realy under the seat is to either:

a) tap into the tail light/number plate lamp feed (blue/red wire) along the LH subframe rail, or...

b) tap into the supply (brown) to the brake light switch on the RHS.

If you go the brake light route, just be wary that you get the supply and not the return (yellow/green) or your GPS will only work when you apply the brakes!

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