Static When Using Multichanger?

Hi Guys, I tried searching for this but had no luck but hope it's not a common question...

I've just bought an 06 Octavia VRS and get terrible static/inteferance when I use the multichanger.

The level of static is worse if the car is running with lights and other electrical systems on, but dies away when the engine is off (although does remain).

My mechanical and electrical knowledge is non-existant, but is this a bad earth?

Cheers!