I can't tell you exactly from memory. I checked all my pics and couldn't find it there either. But I do recall it comes off one of the screws that joins the right side of the cluster (looking at it from drivers seat) to the centre speedo part. There are ground traces behind most of those bolts and the metal back of the cluster is at ground.

It is a common "upgrade" to add a cluster ground. If you look at your 84 cliuster, you should be able to pick which screw to use. I have a ring connector on cluster and then a spade 1/2 way to firewall.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5