Yea. I also looked it up and new parts are really really expensive.
So like I said erlier I'm going to just make my own. Code it base it off Linux. I have a freind who is good with code I'm decent and I'm really good with hardwhere are wiring.
You'll find it easier, quicker, and most likely far less costly to get one of the aftermarket 7" 2DIN size units running Android with GPS and a backup camera than rolling your own system.
Everything is in the one box, with connectors for power, speakers/amp, radio and GPS antennas. Many also can connect to the CAN bus to use the steering wheel controls but of course an old squarebody doesn't have that.
These don't need a connected smartphone for anything.
Simpler and cheaper are headunits that have Android Auto and/or Apple CarPlay. On their own they have limited functions. Connect a smartphone and they use the phone for GPS and many other functions.
Even more simple and lower cost are headunits that do MirrorLink. How cheap? I've seen them around $50 on Temu. Essentially they just mirror the phone screen to their display and have connections for speakers, might have an FM tuner, perhaps a backup camera or other things. They rely fully on the capabilities of the connected smartphone.
Of course there are tradeoffs with these three. The "all up" Android models don't need a phone, but if/when the Android version installed gets past its prime and apps/services start abandoning support, it becomes a paperweight, like anything running Android versions before 5, or iPhones without 5G.
The simple MirrorLink models should be good forever, as long as the smartphones support whatever method is used to connect to them. Change your phone and your vehicle's infotainment also gets the upgrade.
Downside of CarPlay, Android Auto, and MirrorLink is the phone has to be in the vehicle and I assume getting a call or doing anything with the phone interrupts using it with the in-dash display.