Hi Katelyn:
I do not have a direct answer for yo as you say the previous owner messed with the wiring. However by inputting your model and year plus wiring diagram in a Google search I found under "images" google heading so many good wiring diagram and all the wiring colors you need to do your fix. Can I point out many stores have a nice kit with a needle point, bulb and ground clip it is probably what you need to follow your wiring and repair your problem. Voltage meter is good but a 12v light and probe will gain you immediate help and free up a hand plus glancing at a meter to read each time.
All gmc wire colors are the same by the way so a year different or so will not matter.
Good luck
Alton