2nd row, with the suction cup mount. Dash (thru windshield) and roof glass don't work (metallic coating). Tried the little triangle window in the 3rd row and got a really weak signal and 1/10th normal speed, but I assume that would cut out once the car starts moving and turning. Plus there's no easy way to mount it there anyway.
Even tried the frunk just in case that was a surprise material, but nope.
So looks like 2nd row behind driver is the choice. Not as ideal as an skyward angle, but seems to still get a pretty good signal. I'll clip the power cord (with some slack to open the door) to the rear air vent and plug in the 2nd row 120v.
It works in my driveway and doing a short drive in the neighborhood, but the real test will be a road trip.