Where would I find the ECM?
 

Hi guy, still trying to get the car started but I think the mechanic may have the ECM confused with the ABS control unit. Where is the Engine Control Module located?

1991 420SEL

Behind the passenger side kick panel, inside the car.

BTW, it is very unlikely that the ECM is your problem as the car will start just fine with it unplugged. Spray some starter fluid down the intake if you think fuel is the issue. If it trys to start, look at the fuel supplY: pump, pump relay, fuel pressure, etc. If the starter fluid changes nothing, look to the ignition.