The original Ecto-1 bumper strobes were driven by a Whelen A413 strobe power supply. They alternate with a Double Flash, and in my experience you will occasionally only get a single flash out of one of the lights - I attribute that to the voltage input.
I had this same discussion the other day. The frame rate of the film makes it look like they alternate single flash, but I have the correct whelen strobe power supply and correct grimes strobes and it only does an alternating double flash.