Spider Speedo Problem

Help please : random speedo on our Spider today. Just noticed that the speedometer started showing random speeds, or no speed at all! The odometer also only racked up 4 miles after about 20 miles of driving. Occasionally the speed was displayed correctly for a few seconds, before switching 0->5->23->3->65->0 and like that for the whole drive. Stopping to switch off the engine and on again didn't clear the fault, which I'm guessing is a wiring or sensor problem, but I've no idea where to look, as I don't know where the speed sensor is. Any help appreciated. Thanks, Ian

I believe the speed sensor is an ABS sensor on one of the front wheels, so maybe start there and work back?

Actually, Lee, I've found it on the gearbox and a bit cooked and crumbling from the exhaust manifold heat I think. Have ordered a new speed sensor and connecting cable/plug. How the bleeping boot release cable has snapped off the rear lock, so now I'm looking for one of those and some handy hints on how to get it through the bodywork from lock to release handle! Now having to squeeze my arm under the boot and fiddle with the right side lock mechanism to get it open! You OK? It's been a while. Ian