I had a similar issue on the 300SEL for as long as I owned it... the defrost would not fully shut, just partially. I would just zip tie it shut in the summer and remove the zip tie in winter. I became tired of this nonsense and had the PBU rebuilt. Same issue.
Pulled the solenoid block and that section plus one other would not energize (one ground, eight total connectors). Firm whack on ground corrected this, for a while. Then back to the same old ****. Bought a new one, $97 later, system works as designed.
Wished I had tried that a decade ago.
