Start with a pair of these, the kind with the inline remote control (I found mine in the street, run over by a car and missing one of the speakers):
Open up the remote portion find the two connections that the center switch uses and connect those two contacts into a factory momentary switch (available at your local junkyard for a dollar or two):
Remove the speaker portions of the earbuds and wire up a standard 1/8 stereo plug. Mount the guts in a little project box and tuck it behind the dash.
Now, plug the original headphone cable (four conductor) into your phone/device and the recently added stereo (3 conductor) plug into the aux input in your car. You now have a factory switch that can be used for play/pause, next track or previous track. If you cheaped out like me and got the base model and hated all of those blanks where switches for the fancy options should be, you can now fill those spots with factory switches that actually do something and don’t look like tacky aftermarket items. At the same time I added the change holder, a fog light switch and a heated seat switch under the assumption that I’ll eventually add those items.
This first version was more of a proof-of-concept kind of thing. The next iteration of this project will run a microphone to the remote portion of the earbuds so that I can talk on the phone or use Siri whilst driving.
I did add a Bluetooth system to my car, but discovered that it only plays through the front speakers, and I can’t control the volume with the steering wheel controls as it intercepts the output from the stereo and uses its own amplifier. I wanted the factory BT setup and thought I was getting it when I bought the fancier stereo on eBay only to discover that I didn’t get the BT module, and programming the computers for a replacement steering wheel with the BT controls might not be possible according to the dealer. Yeah, I wanted the fancier version of the car, and I wish I had a 2013 instead of the 2012 I bought (2013 had USB/iPod control standard), but I wasn’t going to give up my manual transmission to get that stuff. Considering that I got $4K off list on a $21K car I really can’t complain all that much.