Recommend a cable please

I've just bought a Garmin 1340 and a waterproof case for Gails bike.
I'd like to hard wire it to the bike but I believe (for some reason) you can't just put the unit straight to the 12V supply as the plug in socket reduces the voltage and therefore can't be excluded.

Can you please recommend the best way to do this and the correct (tried and tested) cable.

Hi,
We supply GPS cables with miniUSB or microUSB connectors featuring a waterproof 12v to 5vdc regulator. They are designed to fit into GPS cases.
See http://RiderMount.com.

They are £17.99

Regards, Colin@RiderMount.com