You are correct that it will not detect the (OBD1)vehicle.Only supports 1996 and up (OBD2). Some vehicles afterwards still required use of the old style OBD1 style connectors to access other systems or even more in depth power train control. On these older OBD2 turds that had thee older OBD1 style connector still,  the OBD2 16pin connector existed  for compliance to new regulations , and seemed only good for  just pulling generic OBD2 codes and data for test lane purposes

 You need to jump pins t and e1 on connector under hood for engine codes.short the pins together and count the blinks.

Sorry, 1990 Toyota is not supported by DS708.