Check engine light stays on

Hi, I am a new Benz owner I recently purchased a 1992 Mer-Benz 300se and it is a beautiful car however I am having some problems with the check engine light.

I have replaced the main harness the one that ws on the car the coating on the wires were brittle and exposed I have spent a couple thousand on this problem and every where I go they tell me they cant figure it out, 02 sensor replaced the next thing I could think of is the ECM. I also have a rattle in the instrument cluster is there a way I could get in there to see whats loose. Help light is driving me crazy.

have you had the codes read yet?  That should tell you why the light is on and then you can go from there.