Q: Where do you find the "key code" on your phone for pairing bluetooth to your new car?

You have it backwards.  You enter the car BT system key code into the phone when initailly pairing the devices.  Consult your car sound system manual to find the code.

Modular747 has given sound advice, just to add the pass code is usually set to default as 0000 on all car stereos i have ever linked to.