Microphone is too silent when G4 is connected to car radio

Dear All,

I have a quite big problem, when I want to make a phone call in my car.

I connect the phone's audio out via a minijack to my car's radio and I want to use my phone's built in microphone for phone calls.
Unfortunately the mic does not record my voice loud enough. When I use the phone's built in loudspeaker it works fine, but when I use the car radio as sound output the mic does not record loud enough.

Does anybody know how to solve this issue?

My Samsung Galaxy Note 1 had a car mode available for this.

Thanks!