3. FM modulator (antenna direct connection). This modulator transmits, but not over the airwaves. You insert the car radio antenna plug into the back of this modulator, and the modulator's antenna plugs into the back of the car radio where it directly injects the signal. Sound quality is better, and less variable, than with other FM modulators. Cost is $30 to $75 and you may want an audio shop to install it. Check out products from Fahrenheit and Scosche, among others.
Pro: Adequate to good sound quality. Draws from car power (no batteries). Con: May need pro installation (but it's not that hard). A handful of esoteric cars may lack the standard antenna jack or it will be in the trunk (requiring extra wiring).