Disregarding whatever the real problem is - test the source's voltage independently, using a known good circuit to the battery negative. And test the ground circuit's resistance. If you get similar results you can step away from the MAF sensor and work on your test procedure. Maybe your meter needs work. Or maybe there's a draw on your 12 volt source that you didn't know about and it's not really 12 volts. Verify/calibrate your tools and procedures.