Find a screw in the firewall or the steel structure of the dash and put it on like a washer. If that loop is too big for the head of the screw use a washer on top of it.

 

The chassis is the ground. I think when I did mine, I extended the wire and used one of the screws near the trans tunnel on the driver's foot well side. I also put in an amplifier just above the radio. It was a smaller $100 one. I had planned to put components speakers in the door, so I needed an amp.

 

Well, that ring terminal is absurdly large. Likely intended it to go under a bolt or nut under the dash. In the end, a ground is a ground. Replace the terminal with a smaller one and use any screw into the metal structure, or splice it into an existing ground wire.

 

The OEM factory radio had a female spade lug in the black jack. Replace this ring lug with a standard sized male spade lug.