The tape is covering the coils of the light dimming rheostat for some reason. Who knows why? Perhaps someone felt it could cause a short when installed.

The unit separates easily enough. Remove the screws around the perimeter and the unit lifts out. Remove the speedometer side (metal plate) first. Clean the lenses with mild, soapy water or denatured alcohol and a soft cloth. Check the lens for a tight fit. The lenses are secured in the case by means of twisted metal pieces. These are very brittle so don't go "gorilla" on them trying to bend them. A dollop of hot glue on the edges works to secure a loose lens. If you decide to use a clear silicone instead of hot glue then allow the sealer to sit for a couple of days to "gas out" before re-assembly.

When re-installing pay attention to the rheostat shaft's alignment. The adjusting knob stays in the housing and you need to slide the shaft into it.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now