Remember any switch light of fan in the truck needs a ground potential so flow can work. The energy can't flow in a circuit unless there is:
a. an energy source capable of creating an electric potential difference and unless
b. there is a closed conducting loop through which the charge can move (Grounds once the circuit is closed)
Since I'm a hydraulics type guy I think of the grounds as the return line to the gravity tank.
No grounds - I have a stopped up energy source. Everyone has their own way of dealing with things.
