need help installing fm modulator

I installed a mobile dvd player and the video is fine,

but I tried installing the fm modulator and it gives a really loud high pitched hum.  I tried different setups (at first i thought it was feedback from power inverter i had it connected to, but i also tried other power sources and same thing happens.)

the sound from dvd does work, but that high pitched humm is very loud too.

I've heard of getting a little static, but this is not that, it's pretty loud and steady.  is it the radio causing this?

it's a sony deck that is connected to a cd changer, (the cd changer is made for the radio).

any way i could by pass something?   maybe the switch to the fm modulator?

oh, i should of told you guys,

I don't know which is a power 12v for the radio, so all i tried was a power inverter in my car and the other source was direct to the battery.

the ground i'm using is similar to my video monitor which works fine, but maybe i should try another ground too?

but ground makes it either work, or not work at all, right?