I've tried three or four and I haven't had a bit of luck with any of them. Last one I bought was from Rat Shack and the guy mentioned it had the highest output wattage on the market. Needless to say, it didn't work worth a darn either. I think the problem with these lie in the FCC's regulation on the output (transmission) wattage. They set the limits so low that your FM receiver in your car can't pick up a solid signal, then when it does, it fades in and out because of all the other crap riding the airwaves.
One of these days I'll tear into one and see if I can't boost the output wattage. Shouldn't be too hard I wouldn't think. After that one might try taking the output of the transmitter, soldering some coax to it and extending it to the cars antenna connection. Haven't tried it but that might work also.
Best bet is to hard wire it to head unit via an auxiliary input. As I remember, FM only has a bandwidth of 30Hz to 15kHz. Most stock car radios will deviate (downwards) from there as they were awarded to the 'low bid' contractor.