Let me see if I can answer a couple of questions here..
1. The Top Strobe bar on the Ecto-1a *did* have strobes to the rear as well as the front. (You can see some frames in the movie in the night shots where the roof rack is clearly lit up by rear facing strobes.) Those bars come from the factory with rear facing strobes. You can also order them with Arrow Bars built into the back and some other options for extra $$$'s.
2. The Strobe bar used *is* Commercially available. Its a Whelen Edge Strobe bar. They have been used on Cop cars since the early '90s. The version used on the Ecto-1a was just a standard configuration for the bar. The only things unusual on the bar is the color of the lenses. But all of the lenses are available. (I was able to get the proper lens colors when I ordered my bar for my Ecto-1a Replica)
Hope that helps,
TheEcto1a