Is it possible?

Understand that a common "switched" or Kettering ignition, which is points, breakerless Mopar, HEI, Ford and a few others and NOT CDI or MSD, The coil NEG is NOT simply "switched 12V" All these systems have a high voltage pulse that can be over a hundred volts on the NEG coil terminal. In fact with a "real" Mopar ECU you can get a marvelous zap off the heat sink/ transistor on the ECU because that is the same as the NEG coil connection

So any circuit hooked to coil NEG that is not "up to snuff" can certainly kill off some or all of the signal