Speedo sensor at the gearbox

[Myros]

Evening all. just changed the supect speedo for a known good one, and no difference. No action whatsoever. Perhaps the sensor is duff. Anyone know eher it lives or what it looks like. I've seen something on the RHs of the box, about halfway up and halfway back, whitish, same size as my little finger, with a wire or cable coming out of it. Is that the so and so?

Any help gratefully appreciated, as I'm guessing speed by the degree of hair-sweep-back experienced by the missus. She says she has other things to do.

85 500SL non-switching 4spd auto