Adding kit in cars - actually a bit dodgy
Many of these things, like satnavs, are suckered to the windscreen. Anything in the "swept area", i.e. that covered by the wipers is an MOT failure, not just at the time of the test (when you can take it off) but anytime that the traffic police officer wants to start writing. It's especially relevant if you've been involved in a bump whether it's your fault or not. Having this sort of thing could make it so in the eyes of the insurers and the courts.
Yes I know world+dog do it with their sat-navs, that doesn't make it legal and I do see lots of road users with these things positioned right in their line of sight, not realising just how large an object it can block from view. Reaching for the thing to change channel if it's not close could also lead to a charge of failing to be in proper control of your vehicle. We used to have accidents as people changed cassette, that sort of thing might make a comeback.
I expect also that there might be issues with reception if they are not "visible" to the transmitter through the glass, and also some of the glass in cars such as heating elements, UV filtering etc can affect reception, so its a sort of no-win situation for many potential users.
And what of non-car vehicles with radios fitted? Radios are becoming more common on motorcycles, and I expect there are other types of users such as boats (both leisure and commercial). How far does DAB go, the trawlermen might need the shipping forecast?