Testing Negative Diode
WARNING: This page is about a different car, the 2000 Subaru Impreza and 2000 Subaru Forester. However, it is still accessible from the selected car via links, so may be relevant.
Check continuity between diode lead and negative side of heat sink. Connect ohmmeter negative lead to diode lead and positive lead to heat sink. Continuity should exist. Reverse ohmmeter leads. Continuity should not exist.