From reading around, I gather that the control box / electronics are different for "just rear" or "both front and rear" parking sensors...
My guess would be that your rears aren't working because you have a "both front and rear" control box and it's expecting both sets of sensors to be connected. Because they replaced the normal bumper (inc sensors I presume) with an aero one (with no sensors), the control box is not behaving - just my theory. Likewise, Steve's rear sensors presumably work because he has the "just rear" control box and has rear sensors properly connected to it.
The whole sorry mess is 100% the dealer's fault for allowing you to choose an impossible spec when you ordered. They either need to replace the parking sensor system for a rear-sensors-only setup *that works* or replace your front bumper with a normal one and working sensors at both ends, at your choice.
Either way, I think you're well within your rights to demand compensation for the cockup and inconvenience. At the very least, I would expect them to put it right at their cost and offer some freebies (accessories, servicing, whatever)...
Good luck! My fingers and toes are crossed that my factory ordered Ecomotive will arrive in a few weeks with all the bits I ordered in full working order.