check your rear defogger grid. it also acts like an antenna (AM mostly)

if the rear defogger grid has a bad segment, the AM reception will decrease.

I had the same problem, volvo has fixed the grid 6 times so far. It has broken after 1 to 3 months each time. I am not sure what the problem is. THey put a metalic paint over the broken segment each time yet it keeps breaking. The reception goes back to normal each time they fix it.

The dealer keeps fixing it for free, but When I was a teenager I had an old Plemon - plymoth lemon. I repaired many parts myself. To fix the defogger, You simply clean the glass with alcohol, use two strips of masking tape to mark off the area where the repair has to be made. Then using the special rear window defogger paint you make several layers of the metallic paint. Let it dry, pull off the tape and it should be good as new.

Good luck with your reception

Neil