I haven't tried this yet as my VX, as always, is still in pieces however my idea is to do away with the radio altogether, it is possible to get some small amplifiers from China which seem more than adequate for in car use and can be used via bluetooth. I use one to drive a pair of old mordaunt short speakers in my garage, plenty loud enough and not bad sound. Attach one of these to an ignition switched supply and use your mobile on a holder to stream and control the volume. In theory it should work although I think most are only two channels but probably adequate for a Beta coupé The brand is 'nobsound' have a look on AliExpress, Amazon, eBay etc. see what you think. Very small so could be tucked away discreetly.