When i did my grandads, it was pet smell, wet dog and sweaty people!
I vacced full interior, then applied AG interior shampoo and scrubbed with a nail brush - then wipe the scrubbed area with a clean micro fibre rinsed in fresh clean water. Do the full interior (Floor, seats, mats, roof and plastics on doors/dash) once that has been done i applied another AG product which is the equivalent to Fabreeze but has more of a Automotive smell... Leave that to air dry. That cured the smell, many things will cover it but tbh best thing you can do is a deep clean.
Jonny.