Batt Alt problems

2000 Mustang - 3.8 V6, I would like to say your the
first site that has cleared my mind the best on my
problem. I will hook a ground strap from my Neg
post to my Alt mounting bracket tomorrow feeling
this may solve my problem,  My Batt checks out ok,
New Alt thats now before I install the New ground
strap,  as I drive the car and use the brakes, turn
signals after about 30 min of driving I notice my
Batt gauge starting to drop to the point if I don't get
to the house right away all my systems start
shutting down, fuel pump or ejectors act like their
starting to quit and the car quits. Made it back
home everytime but once, was in luck had my
charger in the trunk an the store let my tap into
their plug to charge my batt back up. Runs fine on
fully charged Batt all the guages and Accesories
work correctly. One other thing question also I
have 2 wires going into my Alt Plug 1 is yellow an
the other blue would one of these be considered an
exciter wire if so which one.