Go to court, tell the Judge that your vehicle did not come from the factory with lap belts, but that you added them and that you were wearing them at the time... and that it's impossible for a CHP to observe a lap belt, especially in your truck which is 4 feet taller than his patrol car.
He pulled you over because he did not see a shoulder harness.
This is total BS and yes a young ignorant whipper snapper, now...were you wearing the lap belt?...