The failure mode of diodes is generally a short (drawing current that is), but they can be open too if they got seriously kazapped. One thing though- if you have a problem and take it to AutoZone for a diagnosis, you end up with roughly the same level of certainty you had before you took it in.

Take it to NAPA or a good alternator shop.

New battery, no run? Something else is going on. Sounds silly, but did you ring out (check with a meter) your fuses and fusible link?

-Bill
'87 4Runner w/ '96 5VZ-FE, 'Red Chili II'
'97 Taco XtraCab 3RZ-FE, 'BlackBean'
TLCA # 13257, Rising Sun 4x4 Club Land Use Coordinator
"He who stops being better stops being good." -Oliver Cromwell