Better than any short-range FM transmitter, you can get a hardwired [FM modulator with Bluetooth] that plugs directly into the antenna port in the back of the radio head unit, then your stock antenna cable plugs into the modulator. The modulator picks up your Bluetooth and taps the output signal directly into the physical antenna circuit at one of (usually) two selectable frequencies on the radio dial; here’s an eBay search for them.

There’s also similar, older-style FM modulators without Bluetooth, just an aux-in jack; these were commonly used to tap aftermarket CD changers into stock factory radios back in the day. IMO these would be preferable if your audio source (e.g. phone) has an aux-out/line-out or headphone jack. Anywhere you can eliminate a wireless signal in favor of a hardwired connection for audio, the better.

Finally, if your stock head unit has a cassette player, there’s also rechargeable dummy cassettes with Bluetooth built-in, so your phone would transmit a BT signal to the cassette adapter, which would then feed the audio signal right into the tape head just like playing a real cassette tape.