Two problems related to power train have been reported for the 1999 GMC Sierra. The most recently reported issues are listed below.
Np8 actuator motor failure.
While driving in snow speedometer started moving back and forth. Then consumer smelled rubber burning, and headlights popped out. Took to dealer and they said something overheated on the outside. But consumer said it was on the inside. Dealer put a module in, and made the problem worse. Consumer fears loosing lights on the road.