I will give it a shot Gm has had some issues with the tail lamp assemblys they resemble a rather large circut board and the metal will some times corrode to the point where there is an open circut right in the tail light itself...If thats not it check the bulbs very closely the fillament may be intact inside the bulb..causing people to think the bulb is ok..but the 3057 bulbs have skinny little wires where the bulbs plug in..and the wire actually gets hot enough to melt right into the plastic plug on the bulb causing an open circut ..I myself see this a lot on city driven vehicles.brake light on long enough to build up enough heat..there are bulbs made without the plastic plug style and have a glass style plug thats part of the bulb, these do not melt..hope this helps..good luck

Check for corossion at rear taillight receptacles