don't forget to check the wiring harness [everything !] by the battery tray. battery outgassing over the years will attack the insulation, and cause an untold amount of hair pulling, until you find the culprit. [especially the harness parts that are hidden.]
if you have those CLAMP-ON replacement battery terminals, GET RID OF THEM ! those cause MORE problems than you can count !
REPLACE the cables BEFORE you begin to even TRY diagnosing the problem !