Mine 89' 420SEL has same problem.

Heat and ac were working but stops in between and restarts again. I didn't bother until it stopped completely and i have to drive it to work instead of my 93' 300E.

When pressing high/normal/low buttons, Can hear small hiss from vents and engine was responding with little vibration but no fan was engaging.

Never did anything myself except changing one rear bulb on MB's.

Was worried that will pay $200 or more at dealer. Tried to search for problem here for week or so. Once got familiar with terms and parts was easier to look around on these forums now.

Mine 30 Amp strip fuse looked intact like new. Touched it and there it is 2 pieces.
Went to Mercedes parts store, bought one for $2 replaced and fan started again. voila...........

Never been happier.
Thanks for all member contributions.
Please update the forum once your problem is solved and what it was...It helps great deal