If you have the Clarion 'Diversity' head unit, it uses two FM antennas, one in each rear window (MW/LW just one on the offside). I had crap reception when I got mine which turned out to be bad connections at the rear of the head unit. I ended up having to have that repaired at Clarion due to it failing altogether later on. They sent two new adaptors back with it and reception's been excellent since. I think it's more likely to be a connection problem you have somewhere, either into the amplifiers or into the head unit. (Clarion charge a flat rate of around eighty quid I think for repairs and they have it back within a couple of days. Absolutely excellent service. They renew any bulbs, renew the display and any other repairs. All the same price. They can also add an iPod lead for an extra twentyfive quid. Really recommend them).
Good luck.