I have a multi purpose cable (bought ex HK) that connects to the 30 pin connector of the iPhone and then splits into 2 with one split allowing you to charge the iPhone via the vehicle cigarette socket and the other spilt taking an audio feed from the iPhone to a 3.5mm stereo plug.

The stereo out cable is then connected to a car cassette deck adaptor (as our car has no Aux input).

The problem is the audio out signal is too strong and overloads the signal onto the tape deck heads such that even with the car audio volume on the lowest setting the sound is too loud and distorted.

Note: iPhone volume controls do not work on the audio out via the 30 pin socket.

Now I am old enough to remember that when cassette decks first became available they only had a Mic in socket for recording, but many people wanted to record radio from their transistor radio by connecting a 3.5mm plug to plug lead. But again the signal from the transistor radio overloaded the Mic input on the cassette deck. So a special 3.5mm plug to plug lead was introduced that I believe had a Resistor installed in it wired in parallel to reduce the signal strength.

QUESTION: does anyone have any idea what impedance that resistor would be - for some reason I have 150 ohms in my head.

Or, does anyone know where I could get such a lead to achieve my requirements of reducing the signal level.

I don't want to use the headphone jack on the iPhone if I can avoid it as this means two leads plugged into different sockets. The lead I have is quite nice as you only have to plug your phone into one cable when you get in the car.