What size object are you trying to illuminate? Full cars? Portions of cars?
If you were shooting Nikon, I'd say get another speedlight to fire remotely, but I don't believe Canon has that same capability without pocketwizards or cables.
Photo by Juan Calderon
The above is a photo that Juan shot with his Alien Bee (intentionally trying to capture the umbrella reflection in the headlight). I'm not sure what version he has. He was gratious enough to let me experiment with it at AMVIV though. The main limitation I see to the Alien Bee system for shooting in parking structures is that you either need to get lucky and find an outlet and have a long power chord or spend more money on portable power supplies. Juan can no doubt comment more on the +/- of the system and what kind of performance to expect from it.
As far as your other option, that would be to get some pocketwizards and speedlights. The pocketwizards use a radio frequency from a transmitter (attached to the camera) to trigger remote flashes that are connected to the receivers. I had an opporunity to shoot a bit with pocketwizards and Canon flashes belonging to Mark Ferguson over Christmas when I was back in Denver. I liked the fact that you didn't need line-of-sight to trigger the flashes, but the combined pocketwizard and speedlight system seems a bit pricey (mind you I'm comparing it to what Nikon's flash system costs to get similar performance out of).
