Inside the car, it smells of oily engine

When driving very slowly, and standing idle, a smell of engine/oil gets into the coupe. (1982 1,5 liter). Smells suddenly as a really old car. I got the car recently, and haven't found any oil leaks so far, but fumes from the engine is very present when the speed is low or when parked /idle. I suspect I have a small leak in the exhaust, not quite sure exactly where yet.
Is the coupe fan sucking air into the car from the engine bay? Where is the intake to the heater/ coupe fan located?
Can it be that exhaust leaks are being fed into the interior through the gear shift lever, if the rubber seal underneath is defect? 

Fresh air gets sucked in through an opening in the rain tray (where the bonnet vents are). There's a rubber seal that blocks air from the engine bay entering that area as well as a number of rubber grommets and smaller seals. There are rubber grommets and seals between the engine bay and the passenger cabin as well - check that they are all in place and in good condition.