My bad on the radio part, thought you had a working one.
Might still be worth checking it with a multimeter at idle then rev the motor and see what happens. If you see it climbing with the revs there may be a point at where the interference passes the threshold of what the gauge can handle and locks up.
I have a cheap meter that i tried this directly on the battery (lowest setting/scale was 200 v AC) when i hook it to the battery(with the ign off) it started at like 2 Volts and slowly keep climbing so i stopped.
Just when out and did it again........27.8 AC volts and holding steady pos/neg on the battery post, ign off?????
