I am not familiar with the 2001 S type electrical system but if it is
similar to my 1990 system I would connect a relay coil between the "ignition
on" 12 volt line and the line from the reverse switch to the lamp module.
These two lines would both be present at the lamp module. This relay would
then energize when the ignition is on and the transmission is in reverse.
The contacts could switch +12V to the sensors. Connect a surge supression
diode across the relay coil like the rest of the relays have. Hope this
helps.
Don Young

Dear Ogglet

I noticed your posting a few weeks ago when I was researching the same
problem for my 2003 s type, and I have found the answer. Contact Ryder relays
( www.rydertowing.co.uk ) and ask them for the towing relay for the s type.
If you fit this then you will get the feeds you need. It cost me around £35.
00 for the relay. I managed to find a wiring diagram (
www.electronicmanuals.co.uk/ )and connected it to the lighting plug on the
rear control module. And hey presto, I have manged to get my reverse sensors
fitted and working. Hope this helps and good luck. Let me know how you get on