Thanks! Your test might suggest that it is somewhat related to eyepoint position... but that would be weird (why not just relate it directly to 3D object coordinates?). Besides, my seat position was all the way back (-100) and the mirror point was even more behind the seat. Very strange, isn't it?
Maybe ISI could explain, how is that related?
EDIT:
It gets more interesting
I decided I will do it in the simpliest way. On the pictures above, mirror pos is right behind door handle. I checked coordinates and it is at about 0.43. I want my mirrorpos at -0.255 (between inside and outside mirrors in Y axis). So, I have to put "-0.685" value for MirrorPos, right? Should be simple as that... but it's not

With that, my mirror position went forward, but at about "A" letter (where 0,0,0 coordinates are for 3D model)... Tell me, how not to get a headache when see things like that? ;-)
Oh, and MirrorPos is not related with Eyepoint parameter. I did some tests and no matter what value I put for it's Y coordinate for Eyepoint, mirror point won't move at all (until I change its coordinates of course).
EDIT2:
Z coordinate for MirrorPos is counted NOT from ref plane (which is the bottom of the car) but from raw coords for the model, as you see it in 3d editor. For example, if your model is positioned like that:
where physical middle mirror is at 0.65, bottom of the car is at -0.3, then if you want to put MirrorPos at middle mirror level, you should put 0.65 as Z coordinate for Mirror Pos (NOT 0.65+0.3=0.95, like you would do when referencing to ref plane).