I know this sounds demeaning, but it happened to me: Have you checked to make sure that your cable didn't break? Mine broke twice in a few months, from being moved around and getting kinked. I should have made a new one from a length of bicycle cable, but I bought a new Volvo one instead for about $30.

If it's not that, I second the theory about the dash-panel wire. On my 245 it was quick to come off, and if you were reaching around up in there you might have pulled it off accidentally. Another thing; is the unit mounted level? Mine wouldn't work unless it was. I'm guessing that there's a mercury switch inside the black box.

Good hunting...