I have a 1998 Eldorado. The upper ac vents don't open all the time. Some times after driving for a while they open. I have checked the vacuum going into the firewall and its perfect. I went under the dash and checked the vacuum line to the canister and didn't get vacuum, I switched the floor vacuum line to the upper vent canister and moved the mode switch to the floor position and the upper vents worked. My question is can the mode switch be the problem?