There are 2 methods I have read about but not tried.
1 is take a microswitch with normally closed contacts, epoxy a launch lug, as a guide, to the switch. When the pin is in place it actuates the switch opening the contacts.
The other , and more simple, method is: use a headphone jack.
Make sure it is the type that interupts the signal to the radios speakers when the headphones are installed. (they have spring loaded contacts that are forced open when male pin is installed)
Use a hardwood dowel as a pin to ensure an open circuit.
You can get microswitches and headphone jacks at Radio Shack.