Do the duct door actuators work off 12V power or vacuum? I had the compressor replaced late last year and I have noticed over the last couple of months that after having the A/C on for approx. 30 minutes that it virtually quits blowing air out of the vents. I've had it back to the shop twice and they have yet to fix it. I think that if it's vacuum controlled that I have some cracked or unattached tubing. ???