i have an '81 280zx turbo and the other night the heater quit working. i have replaced all vac lines and it worked that night and then it got down to like 5 degrees and the next day it didn't work. now it only works sometimes. when it's not working only one of the two pipes going into the dash is hot while the other is cold, like nothing is going through it. what would cause this to happen only sometimes? has anyone had this problem? thank you for any help you can give me. bama16