You just said you tested the alternator and it's not giving enough voltage.
My guess is you need a new alternator, that's what I had a couple of months ago.

 

hi, have a 03 TT that needs a new alternator by what i've read...
i checked this ebay seller;
http://cgi.ebay.com/ebaymotors/NEW-P...Q5fAccessories

but i read here that the porsche # is 996-603-012-02 ? is that for the 6MT version?
i have a tip so, maybe the one above will be ok ?

any other sight to get a descent quality alternator /regulator for a 03 tt x50 tip ???

james R