What you want to avoid is anything that generates electromagnetic noise and anything that blocks signals. Basically, that works out to staying away from the motors and the motor controllers (noise) and not putting the radio inside anything that might be described as a metal box (RF shielding). Higher is better, more exposed is better (within reason). Lexan shielding isn't a problem.
Putting the radio on a metal belly-pan with all the motor controllers, directly between two drive motors, and under a shooter motor and sheet metal shooter deck is the worst possible position I can think of offhand. If this sounds like what you've done, you should really think about making a change.
