The car has a single reversing light in the bumper. The bulb doesn't light at all (tested, also tried spares). Measuring the voltage at the socket I get around 9V with no bulb in the circuit (with just the ignition on; 10V with the engine running). The meter is high-impedance. All the other bulbs are fine.
With no other path to ground in the circuit that I know of (no reversing sensors powered off it) I'm puzzled as to what can cause this voltage drop. The voltage at the appropriate fuse is the normal 12-13V, higher with the engine running. So I've reached the conclusion that there must be a partial short in the switch, but this seems unlikely.
This is a 2002 Focus (UK spec) the 2 litre version with the MTX75 transmission. This means the switch plugs in to the top of the transmission housing, and is a little fiddly to get at but shouldn't be too bad once the air filter housing is out.
The ground at the socket is good, measured to the earthing stud inside the rear wing (where the wiring goes anyway; that looks good), and also to the exhaust pipe. In both cases I see the same resistance as just touching the probes together (0.6Ω).
Before replacing the switch (which will take a few days to arrive) is there anything else I should test?