In the PDF file of the manual! There are some checks for cruise control.
In the case of the car that I dismantled, it proved to be a bad connection in the switch on the steering column. A contact had physically broken away from the switch and was trapped in the multiplug. It made contact but only just.

There is a Suppression r on the chassis arm, adjacent to the alternator. Is it connected?

I saved the gearbox from the 1993 4.2quattro. Currently for sale.

85 WR quattro
90 V8 Silver, 91 V8 Black, 93 V8 Lago
93 100e quattro V6. Titan, i think