Ok heres the deal. I have a 94 C220 benz. Having a bit of an issue. Turning into a nightmare. It started with a miss and I replaced the coilpacks due to the spark on cyl 2 and 3 being weak. The harness to the coil packs was becoming bare due to some good wires that were used and the insulation flacking off. It seems that benz used biodegradable wire on it's harnesses during a few years and I have one. Replaced the ignition wires and plugs. Next I found that injector 4 wasn't firing and no change when it was unplugged. Looked at the wires to the injector and the insulation was turning to dust. Redid the wires to all 4 injectors by insulating them as far back in the main harness as I could. A couple of questions. How can I test the circuit goin to the injectors? Using a test light doesn't work on any of them. How do I test if the injector is bad? Are there fuses or relays in the injector circuit? Plugs are firing well.

 

Sounds like your harness is in really bad shape. I think you could be "chasing your tail" until you go ahead and replace it. There are posts from people on the net who have done this themselves. You just need to set aside a full day and work slowly and methodically.