Update:
Driver's seat removed and base rebuilt with foam installed instead of horsehair for padding. All calico (the padding in the strips of pads) was replaced and the small rips were repaired.
Becker head unit has been replaced with an Alpine CDA-9883E, with full iPod control. Created a custom harness to interface the OEM cabling into the Alpine cabling, to retain complete OEM plug and play compatibility:
Front speakers have been upgraded to aftermarket 4x6 inch drop in replacements. Whilst the new speakers do not have the same mounting design, I was able to fit them with absolutely no drilling, cutting or modifications and there is no sign of replacement. Merely a friction fit in the original space. All original connectors have been preserved.
These cars originally had a 2 channel output from the Becker head unit, fed into a fader wheel near the electric window switches, which then fed the 4 speakers. I removed the cabling, stealing the connectors for reuse, and have configured all 4 speakers to run from the 4 channel output on the Alpine deck. Have left the fader wheel in place, to avoid having a hole in its place.
Rear speakers are CRAP. They are a simple 4x6 speaker, however the cage is custom designed to fit into the parcel shelf using a lip around the edge. Makes replacing with aftermarket 4x6 speakers impossible.