I'm no electronics major but I think the output (impedance?) for the
speakers would be way too high for headphones.
btw..., don't know about in TX but in CA driving with headphones is
illegal. Of course, if you were just a passenger it would be ok.

Headphones while driving is a REALLY bad idea, it's probably as bad or worse
as having the music blarring obsenely (to the point you can't hear sirens
etc).

Even the walkman and headphone manufacturers will recommend never to use
them while operating a vehicle...

And how are you going to wire in a jack, the amplified signal for the main
car speakers would smoke the voice coil in a headphone... and if you can tap
an unamplified signal, it might be too quiet - and possibly not affected by
the volume knob.

What wrong with turning up your music a little louder with the windows down?
If you are looking for sound quality while driving, you might want a newer
car to start with - one with functioning A/C - audio quality and cars don't
really go hand in hand (it's difficult). With our 960 wagon the radio is
decent enough to listen to with the windows down, can't imagine a 740 would
be much worse (I remember our 740 to have a similar sounding radio).