A wide choice of DIN, cigarette-lighter type and dual purpose plugs, fused and un-fused. ALSO extra sockets of different types not to mention cigarette-to-DIN converters (possibly your best option, also allowing you to use phone chargers etc on the bike!)
I purchased extra sockets from them for heated jacket.
I'm trying to do exactly the same. If you get one of THESE you should be able to use it on the bike and in the car - the red cap at the end just twists off.
A friendly word of warning though - when you unscrew it, if you're not careful, the bloody thing falls apart into about 4 small bits. I tried for 20 mins to put mine back together last night, before leaving it for another day.
check out Vehicle Wiring Products in Ilkeston. They have a website and online store.
They are dead cheap and do a plug that converts to BOTH sizes, so you can use it on the GS and in the car.