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.