VW Polo 1997. Poor radio reception. Please can someone advise?

[The Seeker]

Hi.
I've got a VW polo 1997 and all of a sudden I'm having problems with the radio reception. Can someone help with a possible 'earthing' problem?
I had a JVC stereo fitted two years ago, complete with the wiring loom adapter needed for VWs. Everything was fine til about a month ago. I noticed that my radio reception wasn't as good and that someone had swapped my car aerial for an old, manky thing. Some people will steal anything!!

Anyway, I bought a new aerial off eBay and screwed it in, but the reception is still rubbish. I'm sure it's something to do with the earthing of the unit. I've looked at the unit and no wires are loose. When my partner holds the aerial, reception gets a lot better. AM is terrible, especially when I turn the engine on. The aerial is on the roof, so the only way (I think) to get to its base and check the earth there is by taking out all the roof trim.

Can someone please advise me as to what the e problem might be? Cheers in advance.

[tom hopkins]

the aerial is not only earthed at the roof it is also eaerthed at the radio so check here first. alsov if you have access to a test meter check between the inner & outer as it has been known for water to get in the cable & short the two cores out.