I had a 1990 N/A and my alternators were dying all the time! Turned out to be water damage due to the stupid placement of the drain hole in the NA engine lid, it drips straight on to the alternator!! This causes corrosion so they fail faster than they should.
This is the thread I posted -
http://www.mr2australia.com/mr2play/tm.aspx?m=107346 There are two easy options to protect your new alternator from the elements. You can put a shield over the top of it like I did, the black piece of plastic under the hose in the centre of the pic is covering the alternator, the second pic shows the hole in the engine lid which is directly above it!

Or you can get a turbo engine lid as the hole is in a different position as you can see below the alternator is nice and protected :)
The turbo engine lid is much lighter as its made of fibreglass and looks great too so if you have the money to spare its the best option IMO. If not the shield costs next to nothing which is why I did it, your mechanic should be more then happy to do it for you so he doesn't have to deal with it again!