2003 GMC Sierra unsure were to go next?

Hello, I have a 2003 GMC Sierra ext cab 4x4 w/ 4.8L engine. I have 86,500 miles on this truck. The problem is this, The trucks reverse lights and interior dome light is not working. I have checked the dome light cancel button, it is in the on (out) position. Also as far as the interior light go the two small map lights near the windshield are working fine. As near as I can tell both functions stopped working at the same time. I have checked all bulbs and I have checked for power at all plugs and bulb outlets (no power) all bulbs are good. I have checked the the 2 different fuses that control these two functions #1, (INTPARK) interior lights #2, (B/U LP ) BACK UPLAMPS AUTOSHIFT LOCK SYSTEM). As far as I can tell these two functions have nothing in common. I am unable to come with a wiring diagram for this truck. The only thing I can think of is that they may have a common ground. I am unsure were to look for a common ground or a tie point that may be common to the two circuits. Any help would be greatly appreciated. Thank you in advance.