Hate to say it... Check out the Phord's
Rode in a spankin new Phord the other day, and they did it right. There is 4 sensor's in the bumper, and no matter what angle you back up it picks it up right away. It beeps faster the closer you get, and when it quits you can't hardly put your hand between the bumper and whatever you backed up to. Make sure it has an ON/OFF switch you you can turn it off for towing. Not much help for picking an aftermarket system, but thought it may help with what to look for.