Adding late model w123 tachometer (3-pin) to earlier model car

[240dddd]

I have an 81 300TD that didn't come with a tachometer. I also have an 85 300D donor car (cali model). The tach on the donor car is the 3-pin style which I believe is the type that did not use a tach amplifier but rather ran off of a transmission bell housing sensor routed through an ECU.

A PO of the 81 300TD swapped the engine and transmission at some point, I don't have any info on the swap, but there is a two prong wire coming off of the transmission that isn't connected anywhere and is just hanging out in the engine bay.

Assuming this wire is the rpm sensor attached to the transmission, doe anyone know if it is possible that I could make the donor tachometer work in the '81 300TD? Would I need to pull the EGR from the donor car to make this happen?