The cruise control does use the brake light circuit for a digital command. The ground through the bulbs is a zero and anything else is a one. If you have a one (high) then cruise is disconnected. Thus burnt bulbs cancel the cruise. Having extra bulbs is of no consequence .
Although the control unit is often the problem, a proper diagnosis should be done. We use MBs tester which plugs into the harness where the control unit would. It verifies: power, ground, all switch positions, brake light circuit, speed signal (turns a light on at 35kph) and most important it has a button that when pressed, with the ignition on engine off, the actuator goes full sweep.
With a wiring diagram all the above tests could be simulated, including the actuator. The actuator is close to a tie for most common problem.
BTW when you ask about the control unit check out rebuilts. This is a high dollar item and rebuilts are available for near half price.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician