Some noob questions about my buddy's '93 S4

[MnTechnik]

My friend's older brother is letting him drive his S4 for a while, and i have some questions about it which hopefully you guys can answer. First question I had when I sat in it was, what exactly does the differential button on the center console do? Shift more power to the rear I'm assuming but I wanna know for sure. Also, the left headlight lens is broken (no idea they were made of glass) Any idea where he could get a new lens without getting the whole assembly for like 500 bucks?

[kakarot]

button is to lock the rear diff. Dont play too much with it, leave it off.
Where to get a lens, I have no clue, I need 2 of them as well for my 1995 90.

[Blk20V]

diff switch allows the rear wheels to spin at a different rate than the front ones, but automatically shuts down over 25mph.
as for the headlamp, you are out of luck, just pretty much have to bite the bullet and pay for a new one, or find an acceptable used one at a junkyard

[KARMANN_20V]

$110 US will get you a pair of brand new headlight lenses:
http://www.034motorsport.com/product_info.php?cPath=25_66_69&products_id=771