Backup warning system no longer works on 2007 Chrysler Aspen

(2007 Chrysler Aspen backup warning system )
working When I was backing up their was a glass window beind the car the warning system started to beep, maybe 4 to 6 times then stopped. Since then it has not worked since? can you tell me how to rest it or make it start working again.
Ed 4-10-2011

Asked by for the 2007 Chrysler Aspen
Most back up warning system issues I have experienced were caused by (unknown or known) impact of the rear bumper, causing sensor damage. A diagnostic test should help narrow down the issue to a specific system or component.
My wife said no impact of the window when this happened. Does the warning system work like a sonar on a ship? Ed 4-11-2011
Doesn't mean she or you hit the window. Could be a problem pre-existing. Remember that people park really rudely sometimes - and could bump into your bumper, causing sensor damage.

I will have a diagnostic test and post the results, Thanks Ed 4-12-2011
1 more answer
my backup senter quit working a while ago the senter was hanging from snow and ice I put it back in bumper hasent work since now snow and ice broke it off wire