i'm trying to get mine working right now too. it looks like like the amp only needs +, -, and the signal wire to the cluster. you may be able to wire those yourself using similar sized wire, unless your concern is using the same wire color as stock (get the whole connectors if you don't want splices, the pins and sockets can be removed intact).

the harness on the back of the tachometer itself has either + and signal or +, -, signal. i don't see why one shouldn't power the tach amp from somewhere in the engine bay instead of using the original multiconductor loom through the firewall; so any single wire of 18-22 gauge could carry the signal from the amp to the cluster, no junkyard harnesses needed....

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'77 240D, 504H, OM617.952, etc.