What does it look like behind the panel and the headunit? I've seen some cars with AUX ports that use a simple 3.5mm connector between the two, meaning you could just use the male end on the other end of the cable to tie into the aftermarket HU AUX port. If it's hardwired, you'll need to trace those wires and determine what is what. There should be three wires for a stereo connection: right audio, left audio, and ground. For a 3.5mm connector those are in a TRS (tip, ring, sleeve) configuration. The tip of the connector is for left, the ring is for right, and the sleeve is ground.
If you remove that panel and can't figure out what wires are for right/left/ground you can build a test rig from a male TRS connector and some wires to check continuity on each wire and match it to either the T, R, or S.