Here's a suggestion I had some success with. I have a TR 6 with a 60A Lucas alternator. It's not an open-cage alternator like the newer high-output, switched regulator alternators on newer cars - but nethertheless it produced whine on my radio (as an aside, I am not sure I approve of switched, rather than analogue, regulators in an alternator).

The TR 6 bonnet is extremely large, and hinges forward at the front. I was told that the whole panel is not very well earthed due to the weediness of the connection at the two hinge pivots, which means that only the two inner wings were acting as part of a decent Faraday cage (magnetic field shield). What I did was bought two large, flat-braided battery earth straps from Halfords, and connected my alternator casing to ground with one, and with the other, crossed the hinge on one side of the bonnet to earth the panel permanently to the inner bodywork. I also checked that the battery earth was properly grounded to the bodywork by grinding off paint around the bolt point for the standard earth strap.

Bingo! No more noise; I took the strap off the alternator later on, and it did not make such a big difference, so I would say a bonnet earth to fully cage the engine compartment and an improved battery ground might help if my experience is anything to go by.

I drove a BMW 316 and a 525 for a while a few years ago, and both cars had radio interference problems, which the dealer could not sort out. They have always been (and apparently remain) troublesome. It seems worse on radios with high-gain pre-amps from the tuner.