This unlocked when I was playing Land Rush - Pro-Truck National Series.
Its the third set of events after Buggy Challenge and Crosskart All Stars. The Pro-Truck National Series conists of 4 sets of races, each made of a 4 lap Semi Final and a 5 lap Final.
I was playing with the AI difficulty on Competent and it was an absolute breeze, I won every individual event without issue. I had upgraded all my vehicle parts to C by this point in the career.