Fuse location for rear view mirror?

The Electrical Wiring Diagram, available by subscription to techinfo.toyota.com, shows that the Inner Rear View Mirror Assembly is powered from the ECU-IG1 NO.4 and ECU-DCC NO.2 circuits, which are also connected to other essential loads like the Combination Meter, Main Body ECU, Driving Support ECU Assembly, and Certification ECU. I advise against removing these fuses, of course.

I assume you wouldn’t be satisfied with turning off the automatic anti-glare function each time you start the car, as discussed on page 297 of the Owner’s Manual (PDF), but have you considered covering the rear-facing sensor, shown in the illustration on the same page, with some opaque tape? This should prevent the mirror from sensing bright lights behind the car, which would stop the automatic dimming.

I don’t recommend any modifications, but if you want an electrical solution, you could disconnect the mirror assembly at connector O7, which isn’t used for anything else and should be accessible once the No. 2 and No. 1 Forward Recognition Covers have been removed. (See the procedures in the Repair Manual, also on techinfo.toyota.com.) Be sure to secure the separated connectors so they won’t rattle.