Can't play music on car speaker by bluetooth?

I have Kenwood DDX7035BT, when i turn on the music from my iphone, it still using the phone speaker, even i already connected by the navigation system via bluetooth. However, the speaker only working when i call someone.

Sounds like your car system only supports (or is only connected to the phone through) HFP (handsfree profile, for voice calls) and not A2DP (advanced audio distribution profile, for music transmission). However, since it's an iPhone, the trouble may be at either end. Try with a different (non-apple) phone and/or read the car stereo's manual.