Javadude,
Your surposed to put a switch in the power supply to the modulator, otherwise your radio won't tune correctly ie: the same problems your having....
The "Hiss" your getting can be overcome by fitting a "Ground Loop Isolator" in the phono leads to the modulator.
I'm currently running an FM modulator and it's crystal clear on TV, DVD and Video....
ROBIN,
You can buy a 3.5 jack to RCA converter from Maplins for a couple of quid, and also a FM modulator for about £45.
I fitted one on a RX300 last week, and it worked perfectly.
You make it sound so easy and I don't have a clue where to start...........
I wonder if my dealer would know?
Robin