My 87 190D has a very odd bizarre blower behavior!
Here is the list of what it does and not:
The blower works ( you can switch speeds too ) with the key on and engine on EVEN IF the pushbuttons are off!?
When you go to the left most pushbutton windshield icebreaker the blower fan does not respond - just goes to where you have the fan speed switch located - no maximum high.
Now here is one more tid bit: The blower fan switch will make a CLICK, then stop blowing and then CLICK again and keep working. And then repeat.
It's a very consistent condition about every 2 minutes, regardless of key on or engine on. As if its on a timer ( ? )
I just got some reman pushbuttons and a used set - all the same conditions.
Tapping on components, wiggling the key or wiring harness has no effect.
Any clues?

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87 300SDL
87 190D 2.5NA 5sp
90 300GD with 606 turbo!
08 GL320 CDI
12 ML350 Bluetec
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Past rides:
82 300SD - the one that started it all!
81 240D
87 300SDL - the first one.
92 300D 2.5 Turbo
99 E300 Turbodiesel
07 E320 Bluetec
12 S350 Bluetec