I'd be suggesting that normally the pin should be positive & the housing grounded, so with the multimeter red lead on the pin in the middle of the bulb receptacle and the black lead to the housing you should be seeing 12 volts or a bit more with the switch turned on.
Another way to check would be to measure the continuity between the housing and any other ground (chassis, neg battery post) it should beep (if your meter has a continuity test) or read 0 ohms (or pretty close to it.)
Dave
