1. Drag out the old Mighty Vac and start testing the Vacuum Pods.
(you brought it up?)
2.Carefully take the screws out of the Top Cover of the Monovalve and remove it...You're looking for a torn or punctured rubber diaphragm.
You can check the two incoming wires to the monovalve...BUT it'll drive
a DMM (Digital Multimeter) crazy BECAUSE the Voltage is a fluctuating
signal (I.E. More "On" time than "Off" gives you a closed valve and no heat.)
3.The CCU (Climate Control Unit) may be Ill (Let's hope not!)
4.If the Auxiliary Water Pump is no good, when you replace it PUT A ONE AMP
FUSE INLINE between it and the CCU to protect the CCU.
5.An occluded Heater Core will prevent Heat to the passenger cabin.
6. How's the Engine's Thermostat? If it's "Locked Open" You'll freeze in
downtown Havana.
A Bad Monovalve always defaults to Wide Open Hot (So you will have DeFrost)
('Could it be only "Partially" bad?... I'm Clueless)
'Ave a look here,the first Pictogram shows an Auxiliary Water Pump #26:
Phil (at the top of the Pages,FASTLANE) stocks a Monovalve repair kit.
They also stock an Auxiliary Water Pump.
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=126035_568&M=116.965&GA=722.324355&L=765.706&CT=F&cat=568&SID=83&SGR=180&SGN=05