Weak Alternator?

[Wolff <quattro200@earthlink.net>]

My '84 4k runs at about 14 volts indicated on the center console
voltmeter until electrical items are switched on. Each item switched on
(lights, ventilation fan, wipers) draws the voltage down a little until
it is slightly below 12 volts. This is rpm independant. I changed the
brush housing and the symptoms stayed the same. Isn't the regulator in
the brush housing? Is the alternator bad/weak? Maybe there is something
else that would cause this? 
TIA,
Wolff
'91 200 tq
'87.5 coupe GT
'86 4kq
'84 4k "LTD"