W126 heat problems- HELP ME

[W126Ken_W.Pa]

Hi all. I have a 1989 420SEL with no heat. I just bought the car and here's what it does. Blower does work on all speeds, but sometimes I have heat when on DEF, and sometimes not. when i switch to EC, same issue. Now, I do have another 420SEL, an 87, and on that car, when I turn the car on, I can hear the vacuum elements "come to life" inside the dash. But I have not heard this on my new 1989. I'm thinking vacuum leak, but everything looks good as far as I can see. What would a defective climate control switch unit act like? what would a defective mono-valve act like? Any help would be greatly appreciated.