Thanks for the replies.

I installed an extra ground wire to the instrument cluster a couple of years ago when the needles would bounce in sync with the turn signal, so I'm confident that at least the cluster has a good ground connection. I'm not as confident that the part I have is for the speedo because it doesn't have the same connection as my current one which is a plug, not a small screw. The wire coming off of that part is the same shade of green as the speedo wire. What is the pressure sender for?

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'81 300SD