Here's some images...
(Is it possible to make it so these images are displayed, rather than just the links?)
https://drive.google.com/file/d/1B0s...ew?usp=sharing
This is nearly finished before cable-tying cables etc.
https://drive.google.com/file/d/1am5...ew?usp=sharing
https://drive.google.com/file/d/1G4s...ew?usp=sharing
https://drive.google.com/file/d/1BoG...ew?usp=sharing
This shows you where to splice the BT audio signals into the TA7764P chip.
https://drive.google.com/file/d/1tBl...ew?usp=sharing
With the cassette mechanism gone, the left hole was filled with a push-on - push-off button that switches between radio and BT, and the right side now has a 1/8 stereo jack in, which goes to the BT unit. In the middle, I re-used some buttons from something, which connect to the BT unit for play/ff/rw.
The BT unit I used (though there are many to choose from) - was one that's near-identical to this. Try to get one that's APTX (or better) and Bluetooth 5.0 or at least Bluetooth 4.2.
https://www.aliexpress.com/item/3279...d=gZjtaYJFeNXt
The other two circuits I added, which you can see mounted on this top panel (replacing the cassette) are: one was a small circuit board I made which had a DC-DC isolating transformer (to limit unwanted noise and isolate the BT unit), plus the other board was an ebay purchase voltage regulator board. These two boards may be over-doing it, but I wanted the BT to have clean, well regulated power, protected from peaks, troughs, spikes and noise that goes with automotive electrics. So far it's lasted 18 months without issues so perhaps this has helped.
I didn't set this BT unit up so it could do hands-off phone calls - so I can't offer any advice about this (I think this BT unit could do this, but it needed a mike connected).
I would suggest getting hold of a Bluetooth unit, and splicing it into the right spot (on the 2.2uF caps at pins 2 and 19 of the TA7764P), and make sure it sounds ok, then after that, consider replacing electrolytic capacitors (with same-spec but good quality) on the main board to improve the audio quality and give the stereo another lease of life. If you are working with the stereo unit out of the car, and powered by a bench power supply (or car battery), you will be able to get the BT unit working before you need to start putting in the power regulation/protection boards - which only become important when the stereo is back in the car. And once it is all working, you can tackle how you will present the front of the stereo with new buttons etc.
There's a fair bit of detail to go into with this, but this is something.
John