08 Envoy 5.3 Draining battery overnight

[88Ironduke]

Normally I'm trolling the 67-72 Truck side of the house but today I find myself in a bit of a bind. My 08' Envoy Denali has been running for a long time without trouble until this weekend. Thursday I went to head out to work, dead battery. Sitting at 9 volts DC, no big deal. Its winter, but the battery is only 6 months old.... So Friday morning I pull the battery only to it find dead flat, something drained it flat overnight. 24 hours on the slow charger, battery test out 671 CCA on a 700CCA battery, not bad. I get the alternator tested and its making 15.1 volts under load, regulator is going bad, not catastrophic yet but looks like a bad alternator right? R2 the alternator and now I get steady 14.5 volts. I get up today and guess what. That battery is just about dead again.

I set up my volt meter to read amps and start diagnosing a parasitic drain. I have about .65 to .75 amp steady. Since I'm solo in troubleshooting I set up a test light, its an 1157 in a housing. I have used it before on modern cars, I'm looking 25-50 miliamps draw for a number, not nearly enough to make this bulb glow brightly like it currently is. I set it up and start pulling fuses/relays looking for a change in the brightness. Long story short I narrowed my problem to interior portion of the chassis harness, the fuse box is under the rear passenger seat looking up at you. Its actually nice but odd place. I pulled all the fuses and relays one by one and never got a drop either visually or on my DVM. But if I disconnected the main power feed to the rear fuse box my light goes out and my current draw is gone. The rear fuse box feeds a myriad of things, rear HVAC, radio, ignition, dash cluster, wipers etc... I can't wrap my head around not finding a circuit on the quick side by isolating circuits.

Penny for your thoughts?

Paul