Section 420. Guaranties to sellers of liabilities of buyers under retail instalment contracts

(a) No guaranty given to  the  seller  or  to  the
  seller  and  the seller's assignee of the liabilities of a buyer under a
  retail instalment  contract  shall  be  valid  unless  the  guaranty  is
  incorporated  in  or endorsed on the contract or identifies the contract
  and specifies the time balance thereof. A copy of the guaranty  and  the
  contract  to  which it relates shall be given or mailed to the guarantor
  upon or immediately after the execution and delivery of the guaranty  by
  the guarantor. As used in this section, "retail instalment contract" and
  "contract"   include   a  retail  instalment  obligation  and  a  retail
  instalment contract as defined in the motor  vehicle  retail  instalment
  sales  act, constituting article nine of this chapter. This section does
  not apply to a guaranty made by  a  seller  or  a  holder  of  a  retail
  instalment contract.
    (b)  No guaranty given to the seller or to the seller and the seller's
  assignee of the  liabilities  of  a  buyer  under  a  retail  instalment
  contract shall relate to any future retail instalment contracts.