Aux Audio input impedance - why some devices might not work

I've found that my phone will not work with my 2008 Prius aux audio connector. Searching the net yields that some devices work, others don't. I've figured out why... Maybe I'm repeating known knowledge, but I figured I would share anyway.

My phone will detect automatically when a set of headphones is plugged into it's 3.5mm audio output jack, and when it is unplugged. It will NOT detect when the phone is plugged into the car.

I took a cable I had that had on one end a 3.5mm stereo jack, and on the other white and red RCA connectors. Plug it into my phone - and the phone will not recognize that anything happened. If I momentarily short the right channel using a 300 ohm resistor, the phone detects the cable and stays in that mode until the cable is unplugged.

So, go back to the car, plug in my 3.5mm cable to the phone, short the tip of the other end with the resistor to fake out the phone, and I can then plug it into my aux connector and play music

So - now to get a little project box and make something that will present the right impedance to my phone to make it think my Prius is a set of headphones...

Steve