Have you checked the pollen filter for small dead rodents?

Maybe it's time to flog it?

I bought 4 cans of air con foam spray from the pound shop got long tubes with them squirt down the air intakes either side of the bulkhead and take the filter out spray it up there and on the filter ,never had any probs at all and use the aircon a lot in summer kills any bugs in the system most people dont bother but i used to work in aircon so i know how bacteria grow in the system !

had a an old Renault in for repair went for a test drive and smelt something acrid

Turned out to be the heater unit mounted in the bulkhead had a tiny leak and would put the vapor into the air circulation ducts had the same thing thing on a Granada owned by a DCI it came in for rough idle fixed that but on the test drive smelt that acrid smell signs were sore dry throat and bad taste in your mouth but the worst thing was bad headaches i told him about it because i had to fix it he said his wife drove it a lot and had been refered to the hospital for investigation into the servere headaches and tiredness she had been ill for 6 months ,

So if you smell something while in a car get it sorted out !

Are you sure the smell is originating inside the ventilation system and not, for example, something like a cracked exhaust manifold leaking exhaust fumes which then get sucked into the system? (had that with an Accord).