I purchased a stant one (new cap) - pain on the neck to install/remove, a little tight 11235 i think its the number (perhaps I was given the wrong size one). I am just going to get the mbz one tomorrow.
The little motor is the auxiliar water pump and it does have hoses that go from the water reservoir (coolant) and to the coolant reservoir
Windshield Washer Heater
http://img.eautopartscatalog.com/live/thumb/P707473971OES.JPG
I think the issue may be that portion of the coolant does go into the washer tank. I did notice the other day that the level in the water tank rose.
Perhaps this other piece of information is related.
A while back the side ac/heat vents (next to the doors) were pushing hot air (with the AC/heat off). I did some tests and it appeared that could be that the mono valve was not working (which is by the battery) as a last resort prior to purchasing the mono valve I reseated all the fuses, and that took care of that, it seemed the valve started working again and no more hot air (even when the ac was off). I see that there is a plastic hose from the mono valve to the auxiliary water pump / washer tank.
So maybe related too?