if you have a newer alternator with a regulator built in, you need to bypass the external regulator on the rad support.
the stock setup has 2 small wires from the alt to the regulator- one blue and one white. one of those should be going to the new alt in the 2 wire plug on the outer diameter of tha alt case, next to the bigger red wire. at the regulator on the rad support, you need to either make a jumper wire on the 4 wire plug from whatever wire is plugged to the alt (blue or white), to the brown (i think) wire that goes to the idiot light, or cut and butt-splice those two wires together and get rid of the external regulator completely to eliminate clutter.
also, make sure you run a pretty hefty wire from the big post on the back of the alt directly to the battery- this wire is the one that does all the actual charging, so it can't be too big. loop the red wire from the plug on top of the alt to the terminal on the back that you hook the wire from the battery to- this wire tells the regulator in the alt how much juice to kick out.
was i confusing enough for ya?
ok, now that i went thru all that- i'll tell ya how i always do it when converting old to new.
bolt the new alt in, with a new plug from Napa or GM for the alt and plug it in. loop the red wire on the plug to the big terminal on the back of the alt, and a big wire from there to the positive battery terminal.
onthe old alt plug, insert a small flat screwdriver into the back of the plug to release the terminal on the white wire, which i stick right into the new NAPA or GM plug (this saves on crimping and leaves one less thing to go wrong later- and it looks stock). cut the blue wire with the old plug still attached away from the harness to clean things up and prevent problems later.
at the regulator, i cut the white and brown wires off flush with the terminal, and butt splice them together with the proper terminal. remove the stock regulator and cut and tape off a couple of wires- there is a rather large red wire that is always hot- so be sure to tape that one off real good or remove it.
that's it- if you do that, you are done and it looks like GM built it that way. on a couple of cars, i've actually unwrapped the harness and removed the extra wires just to get rid of them, and re-covered the harness with convoluted tubing that i've pirated off newer cars i've junked out.
you don't plan sincerity.
you have to make it up on the spot.
wanna hear about 20 years ago when i was too smart to know any better?