Given a variable distance, r, between Mars and the Sun what is the
equation for the solar insolation on the cross section of Mars
perpendicular to the incoming solar radiation.

Due to the low eccentricity of Earth's orbit, the outcome of this
equation for the average distance to the Sun is known as the Solar
Constant. Mars, however, has a higher orbital eccentricity which
causes an additional forcing factor for Martian seasons in addition to
obliquity.

This may help:
r = a(1-e^2) / 1+e cos(Ls)
a = semi-major axis of Mars
e = eccentricity
r = distance to Sun
Ls = solar longitude, 0 = vernal equinox

Also,

Diameter of Mars = 6770 km