Whatever the mechanic told you, that's the WRONG alternator. I hope the shop isn't too far off, your battery is paying the price. Parts store may have no idea how to help you, either. I had this exact problem with my old '84. Had a two-post alternator in it when I bought it, wouldn't hold a charge. Discovered the thin 3rd wire hanging loose. "Forget it, it's probably for a tach or something..." WRONG! Guys at Autozone: "Duh, the computer only shows two different types..." WRONG! Finally had to crawl around the local boneyard for hours 'til I found the right one. (Found like FIVE variations) Take your car back (bring cables) give them the boatanchor they sold you, ask them to rebuild your old one.

And thanks, guys, for explaining what that wire does... wish I'd known about the 'board when I had my '84.