It's called the diagnostic connector. If there's no electronics inside the cap that covers it, you have the new system.

How do I get one?

Would love to get one of these if you still have them, or even just the schematic so i can piece one together for myself. I'm pretty handy with a soldering iron.

Me, too...
 

A little off topic, but I might need a new one, too. I’ve had one of these for probably 10 years and it’s worked great.

It might have been damaged when I hooked up a Noco Genius Boost GB40 to jumpstart my car. The tachometer was then pinned all the way to the right (>6,000 rpm) afterwards. I removed the unit from the plug after that.

What is the output voltage of the booster? If sub 15 V it didn't cause damage.

What does cause damage is trying to crank with a depleted battery that causes the starter solenoid to chatter. This impresses high voltage spikes on the electrical system and will damage parts. Think about how an ignition coil works. The car owner then blames the booster / battery charger when the damage really occurred prior to the boost.

97SL320:
Voltage not listed in specifications in owners manual, but my guess is that it’s not >15 V. What you’re saying makes sense. Thanks for the input.

Belated update
 

It turned out that the alternator was kaput, so I replaced it with a new old stock Bosch alternator. After driving the car around for awhile to ensure the battery was charged up, I plugged the Techguy tach amp back in, and it’s still working fine, 2-1/2 years later.

Thanks for the update.

Do you have the red one or the green one?

My guess is that it was either picking up noise off the alternator and putting out false triggers, or it was in a diagnostic mode indicating a bad battery voltage level. The red version of the board had a lot of extra circuitry on it to do diagnostics.

Good to get it working. A person with the red board recently bought my replacement amp. He was having the same issue. I never heard back so I assumed my replacement fixed it.

with my w126 diesel, it was the socket, where pin's connect, that was the problem. as it aged you needed to push pins deeper, by a block of wood under the female plug. best fix would be do away with socket, or some how gold coat pin's and sockets.