If the OVP tested as good then you should be able to remove the jumpers from your plug, reconnect the harness to your EGR computer and your Tach should be now be working.

I think the ETR switch is the "anti-ice" switch at the cabin evaporator. It has a capillary tube which you slide into a tube which directs it to a spot beside the evaporator. When below ice temperature, it opens to disable the AC clutch. I replaced it with a very similar one for a 1970's Chevy, both made by Ranco. One hole in the bracket didn't line up so I used a short drywall screw (sharp) into the plastic duct box. I later found some new ones cheap on ebay (liquidator) that I recall have the correct mounting holes.

I also dealt with a failed over-voltage relay. That was a kludge to protect the EGR box. I just left it out and the EGR box too, jumpering across the terminals on both cables to pass the raw tach signal to the dash and then AC clutch relay. I used drawings found on this site. I don't recall if my 1984 or 1985, but I recall in both if you lose the tach display the AC won't actuate, but feel free to correct me.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans