The speaker out, nto the headphone jack.
You may need to boost the signal scrength with a line converter or a pre-amp.
I am pretty new to carputers and I was wondering if I could just run a headphone wire from my carputer to the back inputs of my HU VIA RCA adapter? Is this possible?
Short answer : Yes
headphones share a common ground between the L and R speakers. your headunit may not like sharing a common ground so you'll have to consider that as a possible obsticle.
you'd most likely need a mini headphone amp so you'll have seperate volume control. (i'm assuming this if for a passenger and not for the driver...which would be really dumb) you can purchase or build a cheap 'cmoy' amp that will take a the pre-amp level outputs of the headunit and convert it to an amplified headphone output with volume control.
search for the head unit with auxiliary in or hack your head unit and put aux-in yourself if possible.