as much ......as.......
This formula can be used to make a comparison of degree or quantity.
In your sentence it is being used to make a comparison of degree with the meaning "to the same degree".
It is a part of the product to the same degree that the tailpipe is a part of the product.
-getting back to your question-
The article "a" belongs to the first term of the comparison.
It is as much A PART OF THE PRODUCT as the tailpipe is A PART OF THE PRODUCT.
examples with "a"
It is a much a part of the product as the tailpipe.
Computers are as much a part of school equipment today as pencil and paper.
Intelligence is as much a burden as a blessing.
Love is as much a light as it is a flame.
examples with "the"
Ghandi taught that interdependence is as much the ideal of man as self-sufficiency.
Violence is as much the product of man's nature as the product of society.
Sometimes "as much" is placed after the first term.
Violence is the product of man's nature AS MUCH AS it is the product of society.
It is a part of the product as much as the tailpipe.
He loves baseball as much as football.