The opposite of the front would be the back. That is because the front is the forward part or location, as of a building, and the back is the posterior part farthest from the front.
The opposite location of "in front" would be "out back" (behind). The opposite of being "in front" (in the lead) would be "in the rear" (trailing).
Commit
Front
....behind/back and antonym would be anterior - in front/front
Behind
a front-up
It is the Outback. (see related link)
opposite front side
the opposite of anterior is Posterior.
back
Anterior. Meaning towards the front.
against, in front of, opposite to