His - Denim Blue A220 AMG Line Premium / Hers - Obsidian Black R172 SLK55
That brings back memories of the "joys" of the liquid traffic light air freshener.
They were generally OK for the first couple of weeks when only the upper hole was pierced. However, once you pierced the second lower hole the rate of evaporation of the scent increased exponentially resulting in having to open the car windows every time you sat in it to avoid choking on the chemical cocktail released
Historically, it was to mask the smell of petrol, oil fumes and - perhaps - exhaust fumes that would seep into the cabin through numerous gaps in the firewall and elsewhere, plus tobacco odours and smells of the damp carpets resulting from rainwater leaks and sweaty bodies caused by sitting on vinyl seats in summer.
Looking back with the benefit of maturity , I'd come to assume that it was more likely to be a case of association. That perhaps I was simply car sick, and that the pervasive scent was somehow locked into that memory chain ?
Good old Pavlov