The optimum place to put the antenna is the roof, but it could be placed anywhere on the truck. 5/8 wave will give you the best performance (just my oppinion) a Larson antenna is probably the the best bang for your buck $25-40. My wife has hers on the tool box in the bed of her truck (mag mount) works like a champ. as for the interferance from the light on the roof there shoulden't be much of a problem with that as most fire trucks and ambulance's have there antennas mounted infront of or just behind the light bars and recieve no interferance.
I hope this was of some help