Can I use this wire

[Gweedo]

Hello All. I’m installing my new stereo. I found a wire not being used in the dash. It’s marked 12v neg aux.  I’m hoping this is a spare negative wire. I used it temporarily to test the stereo n it worked. 
Just want to confirm I can use this as a negative ground. The stereo says ground to chasis.   
thnx

[Craggar]

You know it’s a ground for sure because it worked for it when you tried it. It does say Aux. on it as well so I would guess it was a ground for an option that your m/h didn’t get.