On high, my AC switches to the defroster but only when I am driving on the highway. From other threads, I think I am looking for a leaky vacuum line that comes out of the firewall by the heater hoses and that leads to a vacuum canister in the front bumper. My only problem is that my front bumper was removed by the previous owner. What does the vacuum canister look like? I don't see a white vacuum line anywhere. My Cherokee is a 1997. Any good ways of isolating this?