Last spring I had to have some A/C work done and they had to remove the ducts to get to the evaporator. All is fine their but, the front section from where the duct goes in to the front section above the dash keeps coming down. It only does this when it is very cold outside. I have put it back 2 times so far. 

Can some one please tell me what to do to fix this? the pieces look like they just slide together and the rubber piece slides over where the interlocking sections connect. The only thing I can figure is when they get cold they constrict but still they were made to stay together.

Hope someone has some ideas and perhaps a fix!!

Chris