Suburban Tail Lights

I Have A1990 Gmc Suburban, When I Turn Headlights On I Have No Nighttime Tail Running Light , But My Brake Lights Work Fine And Turn Signals Work Fine Any Help Would Be Appreciated , Cant Figure This Out.

!990 style Suburbans are notorious for broken electrical wiring at or near the taillights. Dirt and other road crap builds up and erodes or tears the wires down. Check from the rear wiring harness plug and follow up to both taillights. You will more than likely find your problem. I would have to rewire back there on my Suburban all the time.
Bob