If nothing happens when you disconnect the 2-prong plug, it's not getting a proper ELR signal. Look for the problem upstream. If the tach and AC work (Klima relay in place and functional), the problem isn't the OVP relay and fuse. Given the robustness of these engines, I'd carefully check the wires and connector at the ELR actuator. I'll bet you'll find a bad connection somewhere.

Sixto
87 300D