ILLINOIS P = Power Unit (Tractor, Truck, Bus) T = Trailer   1977 1978 1978   1979 1979 1980   1981 1981 1981   1982 1982 1983   1983 1984 1985   1986 1986 1987   1987 1988 1988   1988 1988 1989   1989 1990 1991   1991 1991 1992   1992 1993 Base 1993 Base   c.1997 c.1998 c.2000   c.2000 c.2000 c.2002