I thought the auxillary circuits (fag lighters, Radio/CDs etc) in trucks were 12V, but if they are 24V I wouldn't put a GPS on it, your battery terminal is likely to push 30V when the engine is running. If you get stuck I'll run up a cheap stepdown circuit for you.
Thanks............Rob, most trucks have an inverter from 24V to 12V nowadays, though they're of the BWM size sockets.
Greg.............D'oh, though it's up in the attic, I thought I'd seen that somewhere............just wanted to check for sure!
Thanks .........you saved me a visit into the attic !