Answer
They have the same magnitude.
The van has the larger net force when the two vehicles are accelerating.
Work Step by Step
The two forces mentioned in the question, $\vec{F}_{car\;on\;van}$ and $\vec{F}_{van\;on\;car}$, are always equal in magnitude and opposite in direction. This is a statement of Newton’s third law.
If the vehicles are traveling at constant velocity then the net force on each is zero.
However, if the two vehicles are accelerating and have the same nonzero acceleration, then the van feels the larger net force because it has the larger mass, and the net force on an object equals $m\vec{a}$.
