Well it sounds like you have tested all the fused circuits, but no mention of the alternator! Take the battery wire off of the alternator and use DMV to check for any voltage between the battery wire and the battery post on the alternator.
CS-Series alternators, use diodes within the rectifier plate known as avalanche diodes. There is an up and a down side to using these devices. On the up side, they prevent damage and surges, current spikes, and such from reaching sensitive electronic equipment inside the cab and under the hood. On the down side, due to the nature of their operation, their lifespan is not as long as the earlier diode trio’s used in 10/12-SI alternators.
In most cases, the only things that go wrong with an alternator, is the brushes wearing out (resulting in a weak magnetic field, this can be easily checked by placing a screwdriver against the alternator case) or the internal regulator/rectifier or
diodes going bad.
When the diodes fail, they usually short out or begin to “leak”, this is when many people first notice battery drains and in the morning the vehicle does not start. Avalanche diodes “collapse” and allow field current to continue to flow, thereby continuing the alternator’s output, instead of shutting down (losing forward bias) and starting up (overcoming forward bias thresholds) which creates surges and spikes. These diodes eventually leak and allow a path to ground, which eventually will drain the battery.
