1992 400E heater not working

[E93067]

Wondering where to start trouble shooting my non fuctional heater in my 1992 400E 160K miles. The blower motor works, and warm air comes out at first , but then turns to cold within about 20 seconds. Another symtom is that my windows seem to fog up slightly after trying to engage the heater. The problem started right after having a valve job. Heator Core? control valve? perhaps some hose was left dissconncted? Don't know where to start and all I have is the very usless mitchell CD-ROM, if someone can reccomend a good CD-ROm or shop manual for this car please let me know.

Any info would greatly be appreciated.