if the switch is on the master cyl, i think its for brake bias.
It uses 5 volt input to send 37% iirc to the rear, it is computer controled and uses G force sens.
As long as decel forces and cornering forces are below 0.5 Gs brake bias to the rear is abt 37% or so.
Once beyond that threshhold the 5volt input is shut and brake bias is restored to 27% to prevent the rear from
overtaking the front

i do know mrec used this sort of tech on ASR cars from late 1995 i think.

As for the polarity i really doubt it makes any diff how u connect it. If one tterminal is wider i would assume its
the live one.
But i guess u know that. Just make sure u use 5volts cause that i do remember clearly cause my mechaninc
refused to install this master on my W124 for fear of me keepin the increased bias active and sendin my ass into some high speed spin.

__________________
Jay,
-----------------
-1995 Blue W202 C36 AMG (M) SOLD ;(
-1995 Black W140 S500 (Lady)
-1992 Black W124 E300 (Dima) (Ex-Mosselman
Twin turbo Kit).
-1988 Black W124 300 E 4-Matic.(Nadeen)
-1983 Brown W126 500SEL.(Old Lady)(Sold)
-1981 Gold W123 280CE.(Dareen)(Sold)
http://www.youtube.com/user/jayrasheed