Send it as a separate payment to your mortgage company requesting that they apply it to principal. Write "Payment towards principal" in the memo section of your check and save the returned check with your mortgage documents (at the end of the loan you may have to account for it).
5.8% over 20 years represents more of a savings than the same over 4 - assuming that your car loan does not have a clause prohibiting or penalizing you for prepayment.-- Mike Harris Austin TX