I've planned to upgrade the stereo on mine but haven't done it yet.  The
first issue you will run into is that the rear speakers use a common
ground, and most modern stereos need a seperate ground for each speaker.
You'll need to either run new wires or use a "common ground adapter".

David Brodbeck, N8SRE
'82 Diesel Westfalia
'94 Honda Civic Si