[LimitedSlip]

Auto-dimming mirrors usually have two photo sensors, one looking forward and one to the rear. The forward sensor determines the daytime\nighttime operation of the mirror by monitoring long-term ambient brightness. The darker it is outside the darker it makes the mirror.

The rear sensor is to detect headlights from behind. If the front sensor says it's nighttime (dark outside) and the rear sensor detects headlights from behind the idea is to further darken the mirror and reduce glare.

Personally I've never had an auto-dimmer that went dark enough or even close to a manual dimmer.

You can try covering the front sensor with tape of varying opacity to trick that sensor into thinking it is darker outside and dimming more. Black electrical tape would provide the most extreme dimming.

Unfortunately, even the most extreme auto-dimming doesn't seem to be as good as a manual dim. If the rear window has a dark factory tint (or more) then auto-dim might be sufficient.

As for the button on the mirror, it turns auto-dim on or off but it's a gradual process and not readily apparent.