1. What do I need to do/build the headphone connection? Can I put
something together using the car radio speaker wires?

You can use the speaker wires, however, the audio used to feed the speakers
will be amplified, and will probably overdrive your headphones. You'll need
to find a way to match the headphones to the speaker output.

On the other hand, if you are using modern low-impedence headphones,
just attaching them across the speaker wires may work well enough.

Of course, you'd probably want to add a phone jack with an extra contact
to cut the speaker out of the circuit when the phone plug is inserted.
With a little thought, you might even be able to add a resistor or two
to adsorb some of the "extra" audio that went to the speaker.

--Myron.
--
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)