If someone can explain how electrochromic mirrors dim, I will be forever indebted.

Here is why. Have always had a wide angle mirror on the driver's side of my e39. As it was, the passenger side and driver's side mirror were identical in size and mounting the passenger side mirror on the driver side required only a reroute of the defogger leads. On my e60, the symmetry between the two mirrors is obviously not the same.

Through realoem.com, I identified a wide angle drivers side electrochromic mirror for the drivers side. Ordered it through my sister (Rome resident).

I received the mirror and dimensionaly, it is identical to the US spec mirror. However, even though I spec'd same build dates in realoem.com, mirror electrical plugs/terminals are different.

Through resistance measurements, I have id'd the electrochromic circuit and the defogger circuit on the mirror. The resistance pattern of the Euro spec electrochromic matches the resistance pattern of the US spec electrochromic.

When hooked up to the car's mirror leads, the US spec electrochromic dims and the Euro spec electrochromic does not dim.

On the car though, I cannot get a read of any voltage change (AC or DC) or resistance change on the lead in wiring.

So, how do electrochromic mirrors dim? I need this in order to diagnose why the Euro spec does not work while the US spec does.