Excellent mod chelboed!
Just wanna clear up a few things though.
BillyP: you can't solder them in parallel, it just won't work. I think the low freq crystal will cancel out the higher freq one.
Nathan: the car isn't crystal controlled, but based on what is called a tuned circuit. This is determined buy R, L & C values in teh receiver stage of the circuit.
In fact I'm trying to design a new receiver circuit that can have multiple freq's.
Check it out
here at ausmicro.com
ph2t.