IPhone Aux Problems in the car?

When I plug my iPhone into my aux in my car, I listen to music but I also want to make and receive calls through the radio. When I do, I'm able to her the other person easily, but I can't talk to them because they barely hear me. When I put it on speaker, it comes out the phone and not the radio.
What can I do without buying another radio or bluetooth?

Try using a different auxiliary cable. Maybe one with the 3 black stripes on the end rather than the 2 stereo auxiliary cable.