Hi
I'd start in the engine bay, its going to be a crankcase breather or rocker cover breathers, there usually piped into the air box or induction pipework, so the engine burns them
at a guess either a hose has perished or come off, then the vapours/smells are being sucker in by the heater vent
with the engine warm and running you should be able to either see it or smell it, if you cant find it, get some damp start and spray it in area around the engine bay, when you find the right area, the damp start will cause the revs to raise, so you know your in the right area
cheers
Joe