Earth/ground point?

Yeap bottom left corner a pillar is a main ground point, hidden behind the removable trim.

There is a ground directly behind where the head unit sits. There's a bolt on the chassis a few inches down inside when the head unit is removed. It's hard to describe but if you shine a torch in you'll see it. You can simply unscrew that bolt (be careful not to drop it off or it'll be gone forever) and attach an earth to that. This is where I earthed both my Xtrons and my boost gauge using a couple of homemade wires with ring connectors on the end that I slipped onto the bolt and did back up. Hope this helps.

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