Problem solved for $1.69.
3.5mm Earphone Adapter Cable with mic for iPhone | eBay
Plug it in, press the button, phone switches on headphone jack until you unplug it. The button is supposed
to act as a Send/End button but I don't care, I'm just glad it works to activate the headphone jack.
Now not only can I use my car stereo to listen to music from my phone, I can also use it to crank the volume on a conversation...previously I've had to turn it on speakerphone and hold it up to my ear like it's not on speakerphone and still couldn't hear over wind/engine noise.
Might want to get a different one than I got, though. The mic on it is really awful. It's still worth trying ones on eBay that are a couple bucks IMO.