After market cruise controll and speed signal

[stein]

I am installing an aftermarked cruise controll in my 85 500 SEC Euro.
The installation is so far straight forward.
Only thing remaining is the last wiring wich includes a speed signal.
The unit can use either a "VSS" (Viechle Speed signal) or a signal from a magnet switch mounted on the drive shaft.
I can not find out if my car has a VSS. I think a magnet switch could create vibrations on the drive shaft, pluss I'll have to crawl under the car.
Any thoughts

Stein