I have tested and replaced many of the throttle actuators and none of the control units. The throttle actuator is an electronic accelerator of sorts. It controls idle, cruise and accel functions. One easy way to test is to see how well the cruise works. An ill cruise system is an early indicator of a faulty throttle actuator on the 104 engine.
I would test this circuit then cut open the throttle actuator harness to see the wires inside. My bet is that they are faulty.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS