Used to be a simple fitting of a capaciter to the alternator,

BUT that was years ago when I used to fit radios.

No idea if it's still the same what with todays tech.

Try a local audio fitters.

MW radio reception needs a good aerial system and a good bond of the radio to earth/chassis.A car of this age will be radio suppression compliant and it is unlikely to need and further components fitted.What make radio do you have?Many of the cheap and cheerful units have very poor MW facility.

Sounds like interference from the alternator. A suppressor fitted on the alternator should cure it, if it doesn't have one already. It's a five minute job to do. The suppressor connects to the main alternator cable and fixes to earth.

Possibly, but I had this same issue few years ago when I fitted an aftermarket radio - it was the aerial amp lacking power. Somehow the FM worked OK (was better when fixed though) but AM just picked up "inteference". Was important to me as I used to listen to LW when in France on holiday. Fixed with an "adapter"

I've not listened to medium wave since the 1980s.

FM and now digital have so much clearer reception - a new radio may be better.

But as said, proper mounting of the aerial and suppresion is needed as a starter.

Stopped listening to MW as soon as I got a radio with FM, that was probably in the early 80's. No disadvantges at all so why bother with MW.