Tach not working on 914-6, Not my car unfortunately... Options

[Nogoodwithusernames]

I'm trying to be of some assistance to the guy in town who is getting his -6 put back together and troubleshooting the myriad of little issues after a restoration. We already figured out that his speedo drive got swapped with a 911 unit at some point so he has the correct one on the way.

The issue at hand is tachometer isn't working. All gauges were rebuilt. I can't remember who he said did them but I recognized the name as a reputable place.

The tach has the purple and black wire hooked up to it, but there does not seem to be a purple and black wire back at the coil. Just the main wire to the distributor, and one wire on each terminal.

Any ideas of what to look for and check? Are we missing something obvious?

[sixnotfour]

@ClayPerrine wrote on Pelican yrs ago,
The 911 tach was originally designed for a standard Points and Coil ignition. The 914/6 used the same tach as the 911. But in 69, Porsche added the CDI ignition to the 911 to prevent plug fouling. That same change was added to the 914/6. Problem is, the tach is looking for the relatively high (12V) signal from the standard point ignition, not the millivolt signal that is being run through the points in a CDI system. So a "tach adapter" or "tach amplifier" was added to the circuit."

I don't have a real picture, but I have a shot from the PET5 program.
it is #11. Part number 901 602 611 00. Official name is the "switch unit BHKZ". It mounts on the 914/6 relay board. "