If had a quick look and found the what you need...

Here's an off-the-shelf 12/24V to 5V 5A step down transformer that'll do the job perfectly and no need for any DIY electronics:

http://www.ebay.co.uk/itm/Car-Power-Supply-DC-12V-to-DC-5V-5A-25W-Converter-Step-Down-Inverter-transformer

Simply wire the input straight into the spare connector and then cut off the Tom Tom's cigarette lighter plug and join the wires left to the transformer's output. The transformer is small, sealed and waterproof and could be mounted anywhere. I'd suggest behind one of the textured panels on the tank, behind the indicators or the ones either side of the radiator.

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