Backup signal not working on '05 Uplander

We purchased a '05 Uplander back in September of last year, but we just recently noticed that the backup alarm was not working when you get close to obstacles. No we didn't hit anything. I have checked all of the fuses and unfortunately, there was not owners manual with it. Does anyone know where I can get a wiring diagram for this thing. I haven't been able to locate one anywhere. Do you guys have any ideas?