I'm not suggesting that you use HAM over FRS but there's a better radio option in case someone decides to do upgrade to HAM at a later time.
Bill, Motorola FRS/GMRS radios have an external dual mic/speaker port that you can plug a headset or a handset into. Ram makes a universal mount for the radio that one could mount outside the vehicle and run the mic into the car. I have one I use stuck up to the sun roofto get it up higher in the car and it works fairly well.
This is not a discussion about HAM Radio being better, but how to get the most out of the FRS since we cannot require all the new members that come and go to have a HAM license, we need an effective way to communicate that most people have.
