Good thread!

I find that the seats and carpets are the biggest contributors to "old car smell". The odd errant McDonalds French Fry can also be to blame sometimes.

My recently finished Datto smells like a "new car", with the smells from the carpet and seats filling the car with the nice smell of freshly moulded plastic and fabric glue.

So it's not impossible to regain the "new car smell".

Dead animals stuck in the suspension are also bad for smells. As are fishing tackleboxes left in the boot on a hot day.

ps.
My old green datto used to stink. It smelt like a pile of sweaty jocks, left inside a warm room for a few days with no ventilation. Everytime it would rain the carpets would get wet, and the car would seriously smell bad. I used to have to put up with smelling like an old man most of the time.