You might need to consider what happens if you join the two outside wires together. Imagine that you are at full throttle, the wiper of the pot in now zero (minimal) resistance from the on wire. If you now short the wiper to the off wire you are effectively
shorting the on and off wires, this might not be good for your radio.

A simple solution might be to use a double pole switch with one pole being a push to break contact this would be wired in series with the on wire.

Hope this makes some sense.
Nick