For those that are interested in upgrading their in-car audio without ruining the classic look of their car. the below are some of the suppliers of suitable equipment.
Vintage Vibes – A US stockist of the Secret Audio SRMS, in the UK this system is available from eBay as well.
Tadpole Radios – This UK based company replace old electronics within vintage head units to build in the latest modern features.
RetroCarStuff – This online company sell vintage look head units that fit classic DIN-D and DIN connector compatible cars.
If you are hunting for a classic upgrade head unit like an Alpine 7909 pictured above then the best place to start searching is eBay.