Heater problems 2001 Jimmy

I own a 2001 Jimmy 4-door and I have been having heater problems lately. It is an intermitnet problem and seems to only occur on colder days (-20 C) It'll blow fine for a week or two then just not work, Sometimes banging on the cover or wiggling some wire harnesses work but not all times. The blower motor has been recently changed, along with the relay, fuses are good. The only thing I can think of is the cold weather must contract something causing 2 contacts not to make contact?? Just wondering if anyone else has problems like this...any help would be greatly apprieciated, THANK YOU!!