Your cruise control is effectuated through your Electronic Throttle Actuator (ETA). I had a 93 400E where the cruise control stopped working and it turned out to be a bad ETA cable (they used the same bogus wiring on the ETA as on the wiring harness). The insulation on the wires inside the ETA cable crumbles and the wires short out. I discovered this by accident because I moved the ETA cable slightly and noticed the cruise control started working again.

Another function of the ETA is Idle Speed Control. I would start with the ETA and cable. Also, if the upper engine wiring harness is original, it probably needs to be replaced. On the 92 model, you probably have the 90 degree angle connector for the ETA. I think they went to a straight connector in late 93 or early 94. Mercedes sells the cable for the old style and, if you are good at soldering, you can change the cable yourself. You have to special order the cable. Most dealers don't carry the part.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles