taillights not to work an a 1998 gmc sierra truck

If your alt. isn't working the engine will draw all power from battery and will quickly drain it dead rendering everything not working incl. the engine. Ck the obvious 1st always. Bulbs, fuses. Also if your tail lights have a circuit board located and attached to the tail lite lense that the socket plugs into ck. with a test light. Or just pull the tail light out making sure bulbs are good, turn light on and wiggle the bulbs. If the light of flicker pull the bulbs and take a tiny screwdriver and insert behind tiny metal connection stripd inside where the end of the bulb inserts. Carefully and lightly push metal strips forward. Bulb back in and they work its that. May have to rplc. circuit board. Pretty cheap. Sounds like just bulbs or fuse.