I suspected some sort of lower WS heating. These will be bypassed as this is a summer only car. The circulator pump will be bypassed too, I'm reading that on some models, this pump can lock, burn the control module and sometimes take the car with it.

Finding out if the electric heater water valve is on-off or modulated is the only question. I'm OK with installing a manual valve under the hood, I've done this on other non MB , non AC cars. Shutting off water makes a huge difference in the summer. Even if the flaps are set to cold, some air leaks past and you still have this hot mass tucked under the dash radiating heat.