Chasing down electrical issues

[C15_Sierra]

Bear with me, I've got some explaining to do...

Before I got my battery hooked up in my truck, I'd been advised to test the circuits with a smaller hobby battery (still 12v, just less amperage) so that I don't set fire to anything when I go to hook the big battery up.

Did that the first time, everything seemed to work except the headlights, etc, and the flasher didn't work. I chalked it up to the fact that I didn't have the 12v input to the lights hooked up (b/c they run independent of the ign. switch), and I couldn't really explain why the flasher wasn't working (the lights lit up, just didn't flash, same with hazards).

Second time I hooked up the small battery, the dash indicators for the turn signals lit up irregardless of whether they were supposed to be on or off. This is with the ign switch ON by the way. Off, everything functions as normal. That's when I started getting the 'uh oh' feeling that I have a short. Additionally, when I had the headlights on, flicking on the turn signals would switch on the seatbelt warning light and buzzer. Weird. Still, I didn't find anything grounds that were suspect

Fast forward to today, I get the big battery charged and hooked up. Now, there is no power going to the cab when I switch on the ignition. I have headlights/taillights/parklights, but no brake lights. Also, when the headlights are switched on, the pass. side turn indicator on the dash goes on and is dim. Drivers side does not come on. All ext. lights are on, except for the R front marker light. Coincidence, maybe the bulb went out, but I remember it working before. Anyway, still no power to the cab with the IGN on. I checked the power splits, grounds under the hood, etc., everything has good voltage. The wire going to the ign. switch sees the same voltage as the one running the headlights, so its beyond me what could be the cause. I don't see why my ign switch would work last week and not work this week...

So, for anyone who's still reading, any suggestions where to start? I'm out of ideas...