Greg,
Heat and vibrations may loosen up that transducer wiring harness over time and result in it coming down, contacting the exhaust pipe, melting the wires and the loss of speedometer signal again. There is no need to ask me how I know that this can happen. 
I just removed and replaced a failed speedometer transducer in my 1990 V12 Vanden Plas last week. I spot tied it into place well away from the exhaust pipe. The failure was not due to melted wires, just another suspected internal failure. I have a box of several bad speedometer transducers in my storage cabinets from our Jaguars over the last 20 years or so. I have heard that they can be repaired if the failure was due to a broken solder joint. I think I will try to disassemble one and repair it the next time one fails.
Paul
I may put a ziptie in there like you said just to be safe. These transducers are getting expensive, cheapest I could find was $110.
