I have a 2003 I35 with 200,000 miles and factory Bose HU & speakers. The HU is getting glitchy with no sound. So I am looking at replacing the HU with a double din carplay receiver. Time to bring the 23-year-old car to modern electronic standards. I read many posts in this AV forum; I am actually confused whether I need the Metra 70-7551 or 70-7550 wiring harness.

I intend to keep my car speakers and the Bose amp which appears to be in the trunk.

Which harness do I need? Looks like the 70-7551 has RCA patch cords but I read low volume problems with aftermarket HU. In that case, do I need a line-level converter especially if 4V/4ch preamp outputs are not available?

I read 70-7550 sounds louder but I also read that the sound is distorted.

So I am hoping that someone can help me to decide. I could buy both since the harnesses are cheap but taking them on and off sounds like a pain.