How can I have my Arduino detect a high signal when the pump is on. I know a voltage divider with a Zener diode will bring down the 12VDC to 5VDC. But the ECU outputs a GND signal. How would I wire this up?
The same way! 
(But, with reversed logic where 12V means is off.)
When the pump-ground is not connected to car-ground, the pump-ground wire should have 12V on it. Do you have a multimeter to check that? If you don't have a meter you can use an LED connected between car-ground an pump-ground (with the usual ~1K current-limiting resistor). The LED should be on when the pump is of
...The pump is like a bit 'ol pull-up resistor. With the ground open, the voltage is "going through" the pump but no current flowing and no voltage is "dropped" across the pump. When you ground it, you get a complete circuit so current can flow and you get 12V across the pump.