Yes, the WRX dimmer controls through what would commonly be thought of as the "ground".
Yes, I can solder, know what the components are and can read the schematic and recreate it. I just won't know the why. I probably don't need to know but then again I "need" to know what and why it works, just a flaw of mine I guess. No biggie, the semester of electronics in high school a decade or so ago didn't get me this far is all.