I'm in a jam, and feel like an idiot....

I'm retrofitting a factory premium JBL indash changer/CD combo into my 1st gen
(thanks to Ricochet for the amp), but I'm missing a connection somewhere...

In the pic, the outputs from the amp connect right up to the existing 89 wiring, but the amp has only one connection to the headunit. What goes into the second connection? All wiring dwgs I've found such as Installdr.com have only the amp output, not the headunit.