Reddit reviews Metra 70-2003 Radio Wiring Harness For GM 98-08 Harness

This. While this harness will work (just gotta splice the red wire coming off your new radio's harness, not the one on the adapting harness, into a new switched ignition wire) it's really not recommended since you'll lose the features and possibly have the issues stated above.

Already have one in the cart, but what is the cable pictured on the left for?

The one you linked is to keep those chimes and radio on until the door is opened.

If you don't care about door chimes and warning chime for lights and key in the ignition then just buy the cheap dummy plug like this one:

The red wire goes to a key-on source in the fuse panel as with gm class 2 data there isn't one at the radio. Most people use the cruise control or the windshield wiper fuse. If you do that, do it right with something like this.

If this is an ls sport, it has amplified rear speakers which I THINK requires a different plug.