Its behind the lower dash cover, drivers side next to the steering column. Get a manual (the CD is fine) for getting the dash cover off, it's a pain.
Usual problem is cracked ground connection -- flimsy construction.
Also, it is rumored that only OEM (Osram) tail-light bulbs should be used -- the control circuit grounds via the stop lamp filaments. The CC turns off when the lamps come one, but if there is too much resistance in the lamp filament, the CC won't work properly. This is a debatable topic (see other posts!), but it is worth checking the condition of the stop ligh bulbs -- if the base is corroded the CC won't work.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!