As above. My guess is:
Red/ white: Feed switched with sidelights
Purple/ red: Accessory feed switched with ignition
Black: Earth
That's not quite consistent with Grubbster's findings but it should be close enough to let you (and the meter) work it out. Please tell us what you find.
The current wiring diagram for "all standard instrumentation, Ford Sigma variants" has these as PR179, RW89, B 15. Let me know if you'd like a copy.
Jonathan
Edited by - Jonathan Kay on 27 Mar 2014 11:52:00