If you are stuck with them, might I suggest you try to cut a deal for some decent radios in exchange for the faulty ones? It doesn't sound as if a failsafe alone will fix your problem. I'm not suggesting anyting fancy, just a medium grade FM setup. In the future you might also go to a nearby hobby shop to see if they can get parts for you a bit cheaper. The hobby shops probably make more money on parts anyway, so they may be willing to get parts for a regular customer. You could also try to bribe the people in the hobby shop with food in exchange for any asisstance. Everyone in my local hobby shop has had a friendly greeting for me after having bought pizza for everyone one time!

Good luck on those truck, I know the boys will love them once you get the bugs worked out!