I'd been meaning to post but it's been over a year now and I haven't...
For about $50 using a FM Modulator and bluetooth dongle (products I used linked below), I made my factory AM/FM stereo a bluetooth stereo while maintaining full factory functionality. I also upgraded the door speakers and did not add any additional speakers. Sounds quality is fine but you can't "jam out" too loudly. While I would love to blast Bad Moon Rising loud enough for the car next to me to hear, I prefer the fit, cleanliness and vintageness of the factory unit!
Plenty loud enough to hear on the road with 4 windows down. I put the FM Modulator on/off switch in the ash tray.
https://www.amazon.com/gp/product/B0007THIDQ/?tag=ihco-20
https://www.amazon.com/gp/product/B00JAPZRH2/?tag=ihco-20