Finding parasitic draws is not fun or easy, but I have some tips:
1- Get one of these adapters (bottom pic), it is specifically made to give you the best test point for the amp meter you need to hook up.
2 - Get a quality volt/ohm meter with amp measuring capability
3 - This video is pretty good for basics, but I'll add my comments about some instructions next
4 - Do NOT hook up the meter directly without the switch, like he does in the video, the reason is simple: when you shut the key off, get out, close the door and lock it, all modules stay "awake" for some predetermined time, usually 5-10 minutes before the BCM (body control module) commands everyone to sleep. What Eric does in the video is power them all back on, at the same time, as soon as the meter completes the circuit.
Why is this bad?
Because (1) if enough draw occurs, there's a fuse in the amp meter that will blow and you're meter is dead in the water until you can replace that fuse, and it may not be easy to find quickly.
(2) What we want is uninterrupted power to the car as we insert the meter into the battery feed circuit. That switch was made for this (worth its weight in gold)
5- It's been my experience that not all fuse boxes are easy to access, especially if it's in the trunk or under the dash, so here's what we do: every door/trunk lid has a switch for an input to the BCM, we need to close those latches to mimic the doors being closed - so they can be left open.
GM has gone to making that switch integral to the latch, so get a pocket screw driver and use it to press the latch pawl in until it clicks closed (as if it was latched to the jamb) This allows us to keep all our doors open while fooling the BCM that they are closed. One note: I have experienced cars that had accident damage repairs that caused the latch switches to not close completely when the door was actually closed, but when closing it with the screw driver, it did close the switch (That one was REAL frustrating to find)
6- Remember any amp load over 10 amps WILL blow the meter fuse, so I advise you to not turn anything on or open a closed door while the circuit is going through the meter. (Can't tell you how many times someone walked up to car I was testing and opened a door)
7- SO, if the car's prepped (draw switch is installed on negative battery post, draw switch is closed, meter is connected to both ends and is on) start it, leave every switch the way it normally is, don't start turning things off (radio, a/c, etc..) we want to mimic day to day operation (so if the radio is at fault, turning it off will just make you chase your tail.)
8- Shut it off, open the driver door latch, close it and lock the car(mimicking leaving the car). Once the outside lights turn off (MAJOR draw, meter killer) We can NOW open the draw switch and start watching the draw go down as modules start going to sleep. After 5-10 minutes, an acceptable draw is 0-100 milliamps, anything higher and I might give it 5 more minutes.
9- NOW the real hard part. Depending on the level of draw, we can sort out what it might be. I ALWAYS check aftermarket electrical parts first, but here is a simple list:
100-500ma - module is awake
500ma - 1+ amp - a motor or light is on
My advice here is look around for lights, listen for buzzing, if the car is cold/cool - place your hand on components and see if they're warm or even hot.
Start pulling fuses, but here's the last bit:
When you pull a module fuse and nothing changes, then plug the fuse back in, the module wakes up- checks in with the BCM - which wakes the BCM up - then they agree they should be asleep, so you'll see the amps go up. Give them time to turn off before proceeding to next fuse.
It's time consuming and requires patience, but when you find that little f**ker, SOOOOO rewarding. HAPPY HUNTING!