You may have some leaking rubber fittings in the PC system. Sometimes the rubber elbows come apart. I believe a service kit is available from FCP Groton. Check in the engine compartment on the passenger side up where the cabin air intake is located. Sometimes the cover warps and doesn't seal well. I've had to use a sheet metal screw on mine to snug it back down to keep engine compartment heat from leaking into the vent system. Also check the o-ring on the oil dipstick. It may be not sealing well. Check the air pump fittings if your car has one. It's connected to the exhaust and could be leaking.

Your flow through ventilation exits out behind both wheel wells. These 6X6 vents have soft rubber flaps on them which are supposed to close when the air tries to enter the cabin. Yours have either fallen off, which happens, or are so dirty they cannot close.
It is possible to reach these vents from inside the trunk. But very hard to get out to inspect, clean them. I have 2 sitting on my workbench which I got off of a pick&pull donor. The rear bumper cover had to come off the fender to wrestle the vests off. I could probably do it from the inside, but it isn't easy.
The flaps are fragile and fall off easily if pushed too hard.

Klaus
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The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)