burning smell at wits end
 

Hi guys .im at my wits end with this..burning smell through vents ..
iv checked everything. .
it doesn't come into the car when its on recirculation. .
and if i turn the ac on it mostly dissappears..
it doesn't start smelling until the car is heated up properly. ..
its definitely coming through the vents. ..
I've checked everything. .brakes ok...
exhaust. ..checked 10 times..no leaks...
checked rear crank seal with borascope to to see if it was burning oil and sucking the smell in the car...I've another post up...not that ..no leak to speak about. ..
There's currently no leak...
The engine is fine..i steam cleaned it..its spotless with no oil anywhere.
iv scoured the site..i didn't run over a plastic bag..and my cat is not on fire..
I've cleaned and removed the pollen filter...even with the pollen filter removed its doing it..
The air flow selector/heater matrix and the fan motor are all in one unit. .im starting to think something around the heater core is giving off the smell because it's definitely heat related ...i blocked up all the exterior vents today. ..still blowing this rubber /plastic burning smell...all vents blocked so ..if i don't find this im getting rid of the car...rebuilt the engine and the car would really appreciate any experience of this..was on a mx5 forum earlier. ..those cars are doing it too..very probable its the same unit in both cars. .they share some parts
no one seems to know what it is ..i stuck my nose right up to the interior vents...its definitely coming out of that area https://cimg7.ibsrv.net/gimg/www.rx8...43eb2733a3.pngCombination unit

Probably a rat nest somewhere. If it started after the car sat from a long time then that might be the problem.