Is it necessary to dispose of a car that was purchased with interest-based installments and fully paid off, knowing that it has become a primary source of income, and is the income earned from it unlawful?

You must offer sincere repentance for dealing with usury, and profusely seek Allah's forgiveness. You are not required to sell the car or stop its income, and there is no blame upon you in benefiting from it and its income once your repentance is sound.