96 grand am engine smell in the cabin

I cans smell engine odors inside the car.
I noticed that my blower insulator cover became brittle and cracked in many places. Is this what causes fumes to leak into the cabin.( my
understanding is there is also another cover underneath insulator that does not look damaged, but it is hard to see without taking the insulator off). Would replacing this insulator likely to fix the problem or are there any other areas i should look into. And how difficult is it to replace it?
Thanks for any help in advance...

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