M20 Tach issues

[10hldh]

Hi all,
I’m looking for some help advice with my tach.
I currently own an 85 318i with an unknown year m20b25 swapped into it by the previous owner.
They’ve wired it in by cutting the body side of the harness where the c101 plug (I believe the square c101) would be and the engine side and then pinning in a deutsch connector.
Talking to the previous owner the tach had never worked but that never bother them. It bothers me however.
I’ve tried running a wire from pin 6 of the ECU (motronic 1.1) directly to the black wire in the blue plug from the cluster but this isn’t working and I’m not sure what to try next.
And help or advice would be greatly received.
Cheers,
Luke