I recently installed a dashcam and, having tucked it tight behind the mirror to comply with the requirement not to obscure the driver's vision, I noticed today that it's partially obscuring this aperture in the mirror housing. Anyone know what it's for?

I reckon I'm going to have to move the cam anyway because the cover of the rain sensor appears quite prominently in the image but I'm finding it difficult to get a good position in the Mk3, not least because of the extreme rake of the screen. I ended up using the self-adhesive mount as it's less bulky than the suction one but now I'm faced with getting it off the screen without damaging the printed pattern. Has anyone successfully done this - I don't expect to be able to salvage the adhesive pad but don't want to mess up the screen?

It's for auto headlights and / or auto dimming of dash lights.

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Thanks Clive, I thought it might be that. That's not a major issue as I don't use autolamps so I needn't rush to reposition the cam but, as I said, I will need to do it at some point to try and get a better view. In that respect the rake of the screen is a PITA when trying to set the vertical angle because the Nextbase mount is also angled in a direction that makes it impossible to point the camera low enough. I've contacted Nextbase about it so it'll be interesting to see if I get any response.

auto lights uses the rain sensor on mine.

It's just for the dimming mirror.   Auto lights is in with the rain sensor as Ian says.

Doesn't matter about slightly obscuring it BUT the light from your camera screen will stop the mirror dimming if it stays illuminated when driving, as long as the screen is off while driving you won't have any issues.

 

Why would the dimming mirror sensor face forwards? I thought it was in the small black spot in the silvered surface to detect headlights behind.

Whatever it is, the cam only partially obscures it and the screen is off whilst driving.

12 minutes ago, mjt said:

Why would the dimming mirror sensor face forwards? I thought it was in the small black spot in the silvered surface to detect headlights behind.

Both sensors are used to control the dimming of the mirror. The sensor nearest the windscreen stops the mirror from dimming in the daylight.

1. The forward-facing sensor detects low ambient light levels and signals the mirror to begin looking for glare.
2. The rearward-facing sensor then detects glare from the vehicles traveling behind you.
3. The mirror dims in proportion to the glare detected compared to the ambient light level.
 

Ah. Makes perfect sense now.

I wish they'd make the door mirrors dip as well. You can get dazzled by glare from them nearly as much.

My 2006 Vectra in Design spec had an auto dipping drivers mirror...one of very few things I miss from that car!

 

Use a piece of dental floss with a sawing action, the black dots are well fused to the glass.