Open f1grafics.cfg. Go to "Enable Mirrors" and set it to "1". Then go to "Enable external view mirrors " and set it to "1" (for onboard only) or "2" for all external views.
This enables the mirrors on all views - incar and outcar. If you cant see things behind you (black or grey screen) it can be that you use a new carshape/ Cockpitshape which hasnt the values included (links to grafics on mirror part...). Otherside it can be a problem of your grafics card. But for that no solution.
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