It really works out well... I bought it off ebay a while back. It was originally intended for my RR, but the connections were not the same, so I shelved it. It was sitting there so ended up putting it into the Defender. The auto dimming part works very well, especially on a triail when the guy behind you has bright lights. It has the compass too, but the doesn't seem to work to correctly.
Forgot to answer your question... I bought it from ScotlandYard...he said it was intended for a RR Classic. Not sure of the brand, but it is pretty similar to the GM and Ford ones. I would probably get one of those, since I think I paid a premium since it was for a RR.
BTW, I did have to turn the connection on the windscreen over to make it work.