You should be running music from your phone too. You want music to pause whenever the navigation is speaking directions, and also anytime you're on a phone call. If you have separate devices, they will try to talk over each other.
Most stock automotive head units have stereo inputs, so there shouldn't be a need to amplify a signal and send directly to the speakers. Furthermore, if the head unit has a tape deck, you can get an analog cassette adapter. Finally, there are cheap FM tuners that accept a variety of input sources and output to an FM channel that you choose.
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