I have a 2008 GMC 2500HD sierra XLT. Defrost doesn't work on passenger side. Any suggestions?

I dont know why it wouldn't work it is all connecter from driver side to passanger side with plastic ducting and tubes . there is a posibility that a tube has come disconnected and if the dash has ever been removed this is very good chance. there should be no blockage or vent do that changes sides. thinking that some one might have done a heater core or some thing and forgot to attack a hose again. I wouldn't fix it if it were me it is a hard job to pull the dash apart for some thing like this. if the window fogs up put it on defrost and flip down that side visor will redirect heat on the window. hope this helps

Thank you for your help