It must be either a flapper valve isn't doing it's job or the dash vent hoses have separated from the vent. I think I have a diagram of the 924/944 heater box setup. I'll dig it out and scan it in for you.

 

I took all the stuff out. Was careful to only remove the A/C system sub harness as you need the ground and relay for for the heater to work and they are mounted way over on the passenger side.

See how much junk came out at...

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