Possibles are the actual control lever that operates it, the cruise amplifier (located under the dash I believe), there is also some connection to the speedometer, and occassionally the wrong voltage bulbs in the rear brake lights with cause a malfunction,as operation of the brakes will interupt the cruise (signaled by the lights coming on). I doubt there is anyone who has owned these cars and has not had trouble with the cruise. Best bet is the amplifier - the soldering cracks.
1995 E420 - retired
1986 420SEL - retired