Almost all FM transceivers that aren't put inline with the actual car antenna are crap. FCC rules limit their broadcast strength severely and even crosstalk from an adjacent FM frequency can be enough to overpower them, or at least seriously disrupt them. Inline transmitters don't have that problem, but at that point you have to pull the radio anyway so you may as well replace it with something that has bluetooth or at least an aux input.

The only time an FM transmitter is a good solution is when you're dealing with things like early 2000s Chevy vehicles, where part of the cruise control module is in the stereo. The best practice for replacing one of those stereos is "add a long wiring harness so you can keep the original stereo hooked up and stashed in the back, then wire the new stereo in to the actual speakers and nothing else."