Did the 1993 Chevy G20 van come with rear mounted center brake light?

No, the 1993 Chevy G20 van did not come standard with a rear-mounted center high-mounted brake light (CHMSL). The CHMSL became a mandatory feature on passenger vehicles in the US in 1986, but its placement varied. While many vehicles had them integrated into the rear window, it wasn't universally located on the center of the top of the rear door on all models in 1993. A 1993 Chevy G20 might have had one added as an aftermarket modification, but it wasn't a factory standard feature.