Extra Part on Ignition System

To make the car's radio quieter, you would install it on the upstream side. Since it is installed on the downstream side, which feeds the ECU, it would prevent the ECU from being messed up by interference from something else, like the CB radio which 67Dart273 mentions. Those were very popular in the 1970's after the movie Smokey and the Bandit. Perhaps the engine stumbled every time he keyed "Breaker, Breaker, ...".