I agree that it's exhaust. Granted the rubber seals around the car (doors and windows etc...) are old but there must be a leak letting fumes out under the car and maybe some of it comes through the vents. You could even have some floor pan rust letting in even more fumes.

edit: I just realized you are in the Phillipines so maybe you have no rust....mybad!

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1982 300D Turbo "Helga"
380,000 miles