GMC Repair: 1999 Yukon SLT 4WD Brake/turn lights, turn signal switch, brake light switch

Question
Hi, The Right rear brake/turn light stopped working 3 days ago. I thought it was a bulb and was going to replace it on Saturday, until the Left rear went out. Checked the bulbs and fuse. Appeared ok, but changed them anyway. Still no luck. The rear upper brake light (high-visibilty) light works so I know the brake light switch is ok. Is there a fusible link that could be causing this? Note: 3 days ago during my morning commute, I smelled a slight electrical burning smell, but thought it was not from my vehicle and forgot about it until both lights went out. Any ideas?

Answer
1. Check to see if the tail lamp socket boars are burnt, if the are then they need to be replaced.

2. If both lower brake lamps are inoperative, but the third lamp works then it is probably your multi-function switch      (blinker switch), because the brake switch sends its signal through the turn signal switch to get to the lower brake lamps.