2000 Corolla radio

Yup that's the radio - on premium models (JBL) there is a small amplifier too. Ever think the problem is the antenna?

Thanks for that - How can I check the antenna? Is it easy to access?

The antenna is plugged into the back of the radio. Look for a black coax cable with a Motorola plug. The cable leads to the antenna mast.

Make sure the antenna connection is not loose where it plugs into the radio. Try gently wiggling the antenna to see if it makes a difference in reception.

Was the radio reception always bad or did it deteriorate over time?

Make sure the antenna is there! It should be near the rear of the roof.

I have seen a couple cars go out without the antenna installed!

Thanks Ray. I'll look at that. The reception was perfect, but then one day, after a particlarly bad rain storm, the FM reception became bonkered - sounds all distorted, though the seek function still finds the channels OK. It works fine on AM. I changed the radio for another one (same model), and can report identical symptoms...

Antenna, antenna cable, or connection is the likely culprit.