Thanks for the suggestion. I've tried grounding to various things over the week. Splicing the parking wire into the black wire. Holding it to a screw with electrical tape. Even soldering it to a ground point behind the radio. None of it worked. From what I've read online, it looks like the head unit is looking for a "pulse" of some sort, which is what lead me to believe the relay was necessary. I followed the instructions with the relay, but no luck.