Here is how I retrofit the original rain sensor in to a '98 Omega.
I replaced the original windscreen for a windscreen suitable for a rain sensor and heat resistant.
Because the prefacelift models do not have this option you have to make the loom yourself.
The starters kit.
Auto dipping mirror for rain sensor. Two relays + feet. The rain sensor + plug. A new stalk. Trim.
New windscreen with rain sensor.
Auto dipping mirror.
Adjusted for rain sensor.
I glued the two relays together and placed them on the base off the original wiper relays (take it apart).
In this way it is plug and play, if you remove this mod and put the standard wiper relay back you have normal wiper function.
Connection plug for the loom that will come from the rain sensor.
You have to make one extra white wire directly from the wipermotor.
The new rain sensor wiper relays.
Remove the old wiper relay and put the new one in.
If you have a wiper stalk with a adjustable wiper wheel you have to make a modification.
Old wiper stalk.
You need a plug (it the same that is in the fuse box).XC plug
Place the wires in 4 and 6 for the right loop.
PeterThe original story (in Dutch)