Cleaning grounds is generally a good thing to do.
I'd do it if working in the area.

But the ground at rear lights serves all lights at that corner of the car. So if the other lights are working on that corner then the ground is OK - at least where it grounds to the body. Could be that the bulb holder is not grounding to the "circuit board" or the circuit sheet is not grounding to its ground wire.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.