Retro or flatpacks...there is still the need that the input to a tach be dialed down. I've not measured what's on the coil- of the Mopar system, but it's hot enough (high voltage) to make you take notice! No way that kind of voltage is going to be running around in a tach.
Pretty good chance there is a similar clamping resistor in any tach.
Now I just have to look inside to see what I can see!
