[wstaab]

Problem is no power to the red/white wire that supplies the tachometer bulb.
The red/white wire is connected to a ground somewhere because is checks for continuity.
It appears that when the instrument lights are activated the power is back feeding through the red/white wire and using the tachometer ground to complete the circuit which causes the signal from the MSD unit to fail. Trying to find the wiring problem would mean pulling the dash which is not an option. I may try to disconnect the red/white wire and splice it to a instrument light feed on another gauge.
Appears like a wiring problem from the factory. Car number 3126 delivered in 2015.
Car has 400 miles on the odo.