Where does the ECU ground wire go in an EA81T? I suspect much of my MAF sensor issues are due to poor grounding,

There's a whole ground wire system, and there's quite a few pins on the ECU usually that go to "ground". What that generally means is they all meet up and get grounded through one bigger ground wire. On the SPFI it's the wire to one of the manifold bolts (or thermostat housing), and then there's another that grounds through the coil bracket/engine block ground at the coolant pipe mount off the water pump. I'm guessing the EA81T is probably similar in it's routing. What I'm saying is you aren't neccesarily looking for a lot of grounds, but probably just one or two where all the ground wires meet up and flow to a single connection to the engine block.

 

GD