Strangely, I've been doing the same on a friend's car.
The problem is that the amplifier and radio are in the boot.
There is a thread here which shows how to put a modern head in and link it to the boot amplifier in the How To section.
My problem was that his boot had at some point turned into a swimming pool and the amp was like a block of rust.
The way I did it was cutting the connector off in the boot area and getting a 9v battery onto the thin wires to listen out for speaker crackling to identify the 4 door speakers and running a multicore from the back to the front.
Works a treat now
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300,000 miles and back on the road
Jobs done: Everything you can possibly imagine ... I have the "Triggers Broom" of cars.