1995 corsica heater problem

Hello there.... I have a 1995 corsica that is having heater problems.... the heater blower works just fine but it only blows outta the front heater ducked and I cant get it to switch to the defrost .... The heater core was replaced right before I bought the car and it also has a leak up near the dash somewhere.... I'm thinking who ever they had replace the heater core did a very poor job putting things back together properly.... Im under the dash trying to locate the problem but it's a little confusing under there.... anybody who has any tips I would appreciate it greatly.... thanks ...Stu

I can't,for some reason, post attachments or I would post a pic of the heater box for you.Sounds like one of the door controls is off,broken or misadjusted.

there is a vacuum block coming off the back of the heater control unit, that runs to the passenger side of the heater core, that distrubutes the vacuum to the proper actuator. If he knocked the vacuum hose(s) off, there's your problem. The two mode valves (one for defrost, one for floor/vent, are located on the drivers side of the center console up high in the dash, so check the vacuum hoses to those as well.