Yeah that "live map" thing Evan said is so true. I did a full stereo install once and thought I could remember which wire was which because the colors were different. Two days later when I had to fix a ground issue, I was staring at a nest of identical black wires. Had to test every single one with a multimeter and it took forever. Now I just put masking tape flags on everything before I unplug a single connector. Saves so much headache.
Factory repair manuals always include wire diagrams for a reason. Labeling each wire makes a live map that helps with any future work. That quick step saves you from hours of tracing later.
Masking tape is good but it can fall off. I use the cheap plastic tags that snap closed. They have a little spot to write on. Wont come off in a tight space or if it gets oily. Stays on until you cut it off.