Hello there Doug - “Been there Done that “ …. Not sure I’m in the position to advise but I will share what I did & why.
First - you are absolutely correct that there is not much room to work on gauge cluster, we had the seats out and the clutch / brake shafts out with the master brake cylinder at the time , so I was able to use the fire wall as kind of a headrest/ support while doing the required yoga pose & stretches to tilt my head far enough back to get the focus out of my glasses , all while having both hands above said tilted head , fumbling away with the wiring connections. 🤷🏼♂️
Next was the headliner, b/c of the overall , questionable shape of ours , we decided to “undo” the left side , front & rear just enough to get at the wiring , nothing to lose sort of thing.
while it is not wrapped together as one unit , it is fastened together in one bundle and then that bundle is clamped in 3or 4 places to hold it in the channel that runs above the door & window on the Cl. Coupe.
Unfortunately, that makes it almost impossible to fasten a new wire unto the existing wire & use it to pull one wire at a time along the side of an untouched headliner into the trunk area.
We were able to reattach the side , front & rear of the headliner satisfactorily.
👍