I had originally a leak at the lid and was able to fix that with an o-ring purchased at an industrial rubber company. It was impossible to seal it with a new MB gasket.
I have cleaned everything after I took the picture, but it never occured to me I could get my finger in there. There is an AC hose running along on the driver side and the belts seem so close to the housing but I give it another try.
I looked at the archives but couldn't find a detailed enough description so I thought I asked again.
There was an excellent write-up of the Monovalve repair by Philip Fox, whom I would like to give credit to as I replaced that today successfully and now enjoy heat.
Thanks again to this group from which I have learnt so much.