My approach is a bit different.
Some time ago I replaced the original head unit with a CDR-220, also replacing the door and rear deck speakers but keeping the stock-style speaker grills.
I then sold the CDR-220 and reinstalled the original Blaupunkt SQR26 because I wanted the original look, someone wanted to buy the CDR-220, and because the original unit sounds pretty darn good. My logic is that my '87 Carrera will never have the "concert hall" sound of my much newer daily driver (which has 450w, 12 speakers, logic 7 surround sound, etc.) and I would have to add lots of acoustic insulation if I was to try.
To adapt the original head unit to "modern audio", I got both wired and Bluetooth cassette adapters (Sony CPA-9 & Ion Cassette Bluetooth Adapter) so I can interface with my iPhone. For the iPhone, I made a removable adapter that tightly fits into the ash tray and provide a location for a "vent" type phone holder.
And yes, that is Miles Davis "Kind of Blue" playing on the iPhone. After all, Blaupunkt translates to "blue dot" and the previous posts displayed "turn blue".