Most systems are plug-and-play so cutting of the factory wiring isn't necessary. Hopefully this is what you have. The p-n-p kits have a plug on the ballast (ignitor, transformer etc) that is the same as the connector on the original bulb. You unplug the factory harness from the bulb in the headlight assembly and replace the bulb with the HID bulb just as you would when replacing a standard bulb with another standard bulb. You plug the ballast into the HID bulb then plug the factory harness into the ballast. Reverting back to the factory setup is simply the reverse.
The PO may have installed a relay harness to power the HIDs off the battery. This means there will be more wiring to disconnect and remove, but still, no wiring should've been cut. The flickering could be a bulb going out or due to insufficient voltage, which is what the relay harness is for. So, you probably don't have a relay harness.
If you're not sure, see if the wiring going to the ballast is the Yukon's factory headlight harness. If not, trace it back to see if it goes to a relay harness. The factory headlight harness should be plugged into that (to trigger the relay) and there should be a larger red wire coming off the battery or fuse box going to the relay(s). Post pics if need be.
