I had the same problem.
An impact screw driver you hit with a hammer did the trick for me.
This is a tool you will need if you do any work on the motor also.
I know it was not corrosion. brand new running wires for lights.
After using a hammer impact screw driver on those screws, I just cut an oil filter access hole in the plastic to avoid a repeat. @trigger Shield covers the hole.
For future reference, hopefully not needed. An impact driver or if not available, valve lapping compound applied to the bit, for more grip on the screw, a firm downward pressure,and vice grips on the shaft of the bit to aid in turning.