>for that matter FM works
Hardwired FM transmitter. https://www.amazon.com/iSimple-IS31-Antenna-Modulator-Aftermarket/dp/B002U5XPBE/ref=sr_1_2?crid=2Z56IV7V3Z3R1&keywords=isimple+fm&qid=1684392573&s=electronics&sprefix=isimple+%2Celectronics%2C203&sr=1-2
EDIT: You said BT, not just aux in. https://www.amazon.com/iSimple-Hands-Free-Calling-Streaming-Smartphones/dp/B00FRU5UNA/ref=sr_1_2?
Yeah, there's a lot, but you may not have many ways to connect one to a factory stereo.
https://www.amazon.com/iSimple-Hands-Free-Calling-Streaming-Smartphones/dp/B00FRU5UNA/ if you don't have any better options
I opted for an iSimple Tranzit Blu HF ISFM2351 FM injector. You can keep everything stock and keep the tape deck.
https://smile.amazon.com/gp/aw/d/B00FRU5UNA?psc=1&ref=ppx_pop_mob_b_asin_image