Use
=SUMPRODUCT(--(Make_Range="Ford"),--(Model_Range="Explorer"),Sum_Range)
replace the hardcoded Ford and Explorer with cells like B2 and C2 in where
you would type the make and model thus avoiding to having to edit the
formula when you change make/model
Ranges need to be of the same size and cannot be the whole column like
A:A as opposed to A2:A100