Nick,
I'm no auto-electrician, but it seems to me that there are two wires for the internal light and two for the mechanism. There's apparently no presumption of an earth from contact with the car body. This theoretically means that it can be used in either +ve or -ve earthed vehicles, so long as the fitting screw(s) are properly insulated (using a grommet or some-such).
But if you know it's -ve earth, then the green/yellow one would be the positive for the lamp, since the other appears to be earthed to the body of the globe-holder and hence the clock-body as well.
The other two must be for the mechanism itself, and the positive terminal would have to be the one that is insulated from the body of the clock. It's a bit hard to tell from the photo but surely one of those terminals must be insulated.
It could be that the different posts on those terminals mean something to an auto-electrician, but as I said, I'm not one
Hope that helps.
cheers
RET