The polarity of the scope is correct. It was grounded to the cigarette lighter, and read +13V when I probed the power lead for the guino.
So, the INJ lead is reading 0 most of the time (there's an underscore next to the A in this image, that marks 0V). Then the pre-fire square wave is +14V for 1000 us (no matter the load, this part stays the same even after I key-off the engine and it decelerates to a stop). Then the first big negative spike, then PWM from 0 to +14V, and a second big negative spike.
