Sorted the cab electrics today. Two things remained to be done: wire the stereo properly so it has a memory, and install the maplight. So I took the stereo out and removed the glove compartment, then found the clock's little power-plug, which has a permanent 12v line to it. Removing the fuse to the clock for a while, I spliced into that 12v line two extra lines: one for the light, one for the stereo. I ran the light's negative to earth, and then put everything back together. Result: one stereo that has a brain, and one nice maplight for the passenger/navigator.