I am out of the loop for knowledge on high end auto music systems

At one time I had also invested money and time in good stereo systems for good cars, but have been out of that market for too long.  I would consider doing an upgrade to the stereo in an older used car I have, which is a '91 Porsche 944S2.  

Where do you feel I should start on my info finding journey?   Mainly for FM reception and multi cd (perhaps SACD} playback.