Rob,
I have a wiring diagram for a 90 Mulsanne S. This is a later, 30,000 series car with Active Ride and ECUs everywhere.
If your car wiring is the same, there should be a 2mm blue-silver wire from the relay to a three-pin connector behind the O/s cantrail mirror (pry the mirror frame outwards from the front; hook retainers at bottom and rear).
My diagram shows a second, 1mm blue-silver wire from this plug to a 24-way plug at the lower O/s A post (by your right foot). From there, it feeds back into the main loom and on to each front door plug (either the 12 or 24-pin, a dashed line to the 12).
The rear window feed connects to a second 3-pin behind the mirror and on to a barrel connector in the headlining above the window. All blue-silver except the final connection which is blue.
There is a black, 2mm earth shown, also above the window.
From the aircon ECU, the relay on my car is fed from pin 18 on the R/H side, which is the main aircon feed from fuse A1. The other side of the coil (ground?) connects to pin one R/H
Happy hunting,
Bill