96 A6 voltage regulator (2.6 V6)
 

How easy is it to replace the regulator and does it require the complete removal of the alternator from the car?

Its not that hard if its the same as mine.
Mines a 2001 a4 B5 2.4 v6, the alternators (A Valeo) under then engine on the right hand side (from inside the car)
Undo battery as the big fat red cables alive all of the time and right beside the dipstick tube.
Remove the plug with one wire on it, try undoing the oil level plug first as its the same design and right close beside it, for practise.
Remove the big nut holding the fat red wire cable.
Remove 2 small bolts holding the black plastic cover onto the alternator.
Remove cover.
Undo the small 3 bolts holding the regulator in place.


If the red battery light came on on the dashboard, amd turned off at some point, and came on again, maybe more times than that.
Its the brushes that are at fault.
A quick fix is to just replace the brushes. Its real cheap but a little fiddly.
But by then the slip rings worn too, and the bearings too.

A good fix requires these all to be done.