Battery Drain/Charge

[zafarhayatkhan]

Car: 94 E320

A few days back, ASR/Check Engine lights started coming on intermitently. Then the car failed to start, clicking noise. Replace battery. Everything was good for a few days.

Then the car failed to start again. Jump started, had the battery and alternater check at Advance Auto Parts: both good. Drove around, came back, started fine. In the morning, the car failed to start again.

Ordered a voltage regulater from Phil Today.

What is the most likely source for drain?

This morning heard the AC blower with the key on, PBU was in the off position. The sound was very faint, so not sure.