alternator on my 1996 2500cc ford transit?

i dont want to buy a new alternator and find the red light still on,even though it is glowing dim. I think it might be the wireing at fault. How do i find out if it is the wireing. i think it might be the small blue wire thats conected to the alternator between the two red wires. can someone tell me what this does or where it gos.