GMC 2001 Envoy - Manual (Page 126)

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Mirrors
Inside Day/Night Rearview Mirror
Pull the tab under the mirror toward you to reduce glare
from headlamps behind you after dark. Push the tab
away from you for normal daytime operation.
Electrochromic Day/Night Inside Rearview
Mirror (If Equipped)
This mirror automatically changes to reduce glare from
headlamps behind you. A photocell on the back of
the mirror senses when it is becoming dark outside.
Another photocell built into the mirror surface senses
when headlamps are behind you.