Automatic Day Night Mirror fix for inadequate dimming

From day 1 I hated driving my Prius plugin advanced at night because of the Automatic day night mirror. It doesn't dim enough automatically to be in any way comparable to every manual day night mirror in every other car I have driven. At first I would carry a AAA penlight and hold it up to the headlight sensor to force it to dim down more when headlights behind got too annoying. While it solved the problem, holding a light up to the sensor gets old fast. After confirming with the dealer that the mirror is working correctly and not defective I pondered solutions. I use the home link buttons and was not able to find a manual day night mirror with home link which would fit. Since I knew I could get the dimming I want by shining a flashlight on the headlight sensor I decided to attach a LED permanently over this sensor and wire it in so it comes on with the headlights.

1) I bought this "Little Dot SMD Accent Light" led: Little Dot SMD LED Accent Light | Tractor LED Accent Lighting TAK | LED Tractor Application Guide | LED Work Lights | Super Bright LEDs

2) I filed off excess material around the perimeter of the LED so it would fit nicely centered in the recess leading to the headlight sensor on the mirror. Glued it in place with JB weld epoxy. Used black electrical tape to cover the lead from the front to back of the mirror and taped the wire to the mirror pivot post after a partial wrap around it to allow full mirror movement without stressing the wiring.

3) To get inside where the mirror wires run I ran the wire up through the gap between the mirror mounting bracket and mount plate attached to the windshield. I chose to connect the black wire to ground via one of the mirrors wires but might not do that if doing it again since that wasn't very easy to do. It can be run along with the red wire and connected to ground elsewhere. The red wire I ran past the mirror wire connector to the space beyond but still under the black plastic wire cover and attached a male bullet connector.

4) Now all I needed was power that goes on and off with the headlights. I tapped into the brown wire behind the left lower dash panel as described below and ran the wire up to the mirror where I attached a female bullet connector and plugged it in to the red wire from the LED.

I got power from the The brown wire that is active when your parking lights or head lights are on. Details for accessing and tapping into the wire begin at step 4 on this page:
DIY: How to add aftermarket OEM-style fog lights to a 2012 3rd gen Toyota Prius Two, Three or Four (non-Five models); complete retrofit installation guide with wiring diagram | Balancing Act

Now whenever the headlights go on this little LED shines brightly on the mirrors headlight sensor telling it to dim to maximum. Problem solved. Now mirror dimming is truly automatic. If for some reason I have yet to discover I don't want the mirror to dim, all I have to do is turn off the mirrors power switch. Home link works either way.

Update 9-7-15: Pics added. It is closer and may be easier to get power from the shift knob illumination light which is located between the dome lights as Maarten28 suggested. The wires leading to it were not visible from the covers I removed. I don't know how you gain access to that wire. It may be a simple matter to remove the roof console to gain access to the lights wiring but I have not tried. Perhaps one of you will figure out how to tap into the shift knob illumination light and report back in this thread.

 

Yes the wires from the LED run up into the headliner. The power to the LED glued to the mirror just runs from the brown wire behind the lower left instrument panel up inside the left front pillar and tucked in across the front edge of the headliner to the mirror. The linked instructions detail how to get to the brown wire which once I knew how the parts came off as detailed the linked instructions was quick and easy to get to. All I did beyond that is pull off the left front pillar cover starting from the top edge next to the headliner. Oh yes there is a plastic cover in the headliner just above the mirror where the mirror wires go into the headliner which I also just pulled off. I don't have any pictures of the mirror because everything is black and won't show up well enough. The mirror looks just like always except the headlight sensor is no longer visible replaced by black tape over the LED and leads.

You might be able to get power from the shifter illumination light. I didn't think about that possibility. I don't know when that light is on or off because I don't notice it.

 

Yes there is power at the mirror but it is preasent 24/7 so the led is on all the time. I used that to test out how well it would work.

I also wanted to open the mirror and bypass the sensor. I wasn't able to figure out how to open the mirror without breaking something.

 

It probably is somewhat subjective what the right balance is. Not trying to talk you into a project or anything by my comments here but I think you raise a valid point and for many the mirror may be just fine without doing anything. I think perhaps some more detail on the effect might be of use in the thread. It is easy to see what the effect would be for yourself before doing the mod by holding a AAA flashlight up to the headlight sensor on the face of the mirror while driving at night. For me, my mirror isn't capable of dimming too much even with my mod. When it is fully dark out the effect is reasonably comparable to the effect of a typical manual day/night mirror flipped up to night position after my mod. Definitely not darker than a manual day/night mirror. I would say it is about the same to perhaps not quite as dim as a manual mirror.

There is still automatic response to dark. The light sensor on the back of the mirror facing out the windshield still does it's thing responding to how dark out it is. When it gets dark to a certain point the mirror begins to dim and is fully dimmed by the time it is fully dark out. My mod just tells the headlight sensor there are bright lights behind so it needs to dim fully rather than the middling dim without my mod which left me half blinded by the SUV behind.

 

The mod is definitely NOT hazardous. I think the effect is the same as a manual flip mirror. You can still see headlights behind you with a manual flip mirror set to night and this is also true after my mod of the Automatic mirror. Easily judged for yourself with a flashlight held over the headlight sensor. I would argue that the Automatic mirror as delivered on the car is on the hazardous side because lights behind you are reflected into your eyes blinding bright unless the split in the rear window blocks them.