Ground loop isloators / noise filters

They work if they're installed in the right place. All they do is block AC voltage from passing through a DC circuit. The AC component is noise. AM radio reception is highly susceptible to interference. There are some tricks to finding the general source of the noise, and often it's because of a bad ground connection somewhere. The good old school installers know how to troubleshoot this problem, so try going to someone who actually knows what they're doing.

what kind of vehicle and what kinbd of stereo. I have seen cheap stereos cause those problems. and some vehicles are more proned to inducing noise.