Finally got round to changing the existing Clarion cassette head unit for a nice Alpine, the sound is infinitely better but all radio stations still crackle like MW. Would it be best to replace the exising fiat aerial for an aftermaket one? I've searched the FAQ's but there is nothing that covers aerials/reception.

I would check the cabling, state of the rear antenna etc first. I have a Pioneer head unit and the radio recption is perfect.

(unless a truck stops too close behind me)

Where do i find the antenna? I thought this was incorporated into the rear window.

Yeah it is AFAIK. i meant take a look at it, see if it's worn at all.

Did I dream it, or isn't there a signal booster thingy powered by the feed that usually supples the electric aerial supply from the head unit?

The aerial amplifier is behind the n/s c-pillar cover, next to the rear screen. The screw-on connectors can work themselves loose and give a poor signal.
Mine was hanging off!!

Thanks for all your replies. Checked all the wiring today all seems to be ok and amplifier is securely mounted too. Reception still rubbish. Could it be the booster which is failing?