Dash Cluster

Please note, Amp gauge is hot at all times. Disconnect neg' battery cable is first step of most any service, especially this one.
Your inst' panel is much easier than some of the others to R&R. After you have removed it once, you might decide its wasn't so difficult.
If you decide you don't want to R&R it repeatedly for one problem after another,,, do everything you can do while you have it out. Clean all electrical contacts. Every little screw that attaches circuit board to housing is part of the ground path. Replace all light bulbs ( LEDs optional ). Replace antique mechanical voltage limiter with new (solid state IVR from RTE best option but pricey). Clean crud from the square drive port on back of speedometer. I wont go into pulling the speedometer cable for clean and lube because I don't know the 66 is made the same as 67 up.
If you don't bypass the amp gauge, at least pull and inspect it. If there is metal debris on its little round magnet, compressed air will remove it. Replacing the amp gauge with a volts gauge may or may not be an option. Some housing designs don't provide enough space to fit a aftermarket volt gauge in them.
Wiper pivots is a totally different section of service manual but... one of them is right there behind the inst' panel. Look for evidence of water leak there.