I mounted mine yesterday:
I took off my kickpanel by pulling off the hood lever, unscrewing the two screws by the hood lever, and pulling off the panel.
I wrapped two zip ties around the PNP harness to start. That way I could have some mounting points for the harness. Next I put on some velcro on one side for mounting. Looking at the stock ecu there is a black rectangular box attached to it. I put some velcro on that box to hold the PNP harness. After that just look for places to wrap a zip tie around and go pass it through one of the zip ties around the PNP harness.
NOTE: Be careful how rigid you put in your harness. You want to make sure that it can wiggle around while you are connecting the wires from and to the ecu. If it is too rigidly mounted, then a short wire can screw up all your work
It's cake after that. The actual piggyback can fit anywhere after that. I wedged it up behind the coin tray. Hope this helps.