So I just bought this HU-615 on eBay in working condition. I got the code from the dealership and plugged it in.

Here's what the back of my HU-555 looks like.
Here's the connector that the main sound comes from. (Right connector).
Here's what the back of my HU-615 looks like.

As you can see, the 615 is missing the connector and as a result, I only get very tinny sound from the tweeters(?) that are on the dashboard.

I verified this using the old unit and testing with and without the connector in.

According to this harness pinout, it looks like this is all I need and there is no mention anywhere about that extra little connector.

I assume that the other connector is just for a CD changer which I don't have and have no interest in.

My question is, what can I do? I'd really rather use the stock radio.

Thanks.