I dont think the E12 have those flaps, they vent out the sides on the c-pillars. ( well i know mine doesnt) I would suggest check the boot rubber is fully sealing and not perished or the gasket seals around the rear lights. If perished either will allow the exhaust fumes to be sucked into the boot and car.
Easy way to check, take masking tape and seal the boot, drive the car and see if you still get the same fumes. If no then boot rubber needs replacing, if yes then tape up around the rear lights and drive again. If the problem still persist then check the rear door rubbers too