Hi,
this is probably and earth fault, to test it connect a very thick wire like mains cable between the battery Earth and touch a known Earth point on the tailgate, turn your rear heater on/off and check if reception improves. Don't put your finger on the bare wires as this may lead to a false result as you will become the aerial and increase the signal inadvertently.
Alternatively do what I did, remove your radio/CD and connect an Earth lead to the body of said device and then connect it to the bolt that fixes the grab bar on the dash as this is a good Earth connection to see if it helps reception, this also get's rid of the dreaded rear speaker buzz/clicking.
I've also done a big wire Earth mod from the battery to the rear body of the car, my FM reception has increased greatly, I'm intending to run a new thicker Earth lead to the hatch next to the aerial amps in the hatch when I get some time to do it.
Hope this helps you.
Cheers
Doug