How to make my FM transmitter work?

I've recently got a new car which had a Cd player already installed in it which is pretty handy although in my old car it had the radio and tape player so it was easier to find an empty channel for my FM transmitter to work for my ipod, in this car the cd player keeps skipping channels and won't find any empty ones and the ones it does the fm transmitter won't pick up as it only goes up to so many, I've read through the manual and nothing please any advice would be greatly appreciated , it's driving me mad!