Yours is a '73, a GT6 Mk3 like mine.
It is really not hard. This summer to look for a short I removed the engine wiring harness from the interior under the dash to the headlights, & since the problem wasn't there, re-wrapped & re-installed it.
It's a little slow, but just thread-the-needle. Take out the seats so you can have plenty of room. You will likely need to spray PB Blast or other rust penetrant on the seat bolts from above & below for 2 days before removing. Replace the seat bolts w/new stainless ones from the hardware store, & put plenty of anti-sieze on them. The seat bolts are pointed, so get them a bit long, run the right size nut or die on them, grind or file them to the same sort of point as the originals, then when you take the nut off it will straighten up the threads. Run the bolt on & off a few times to make the threads perform nicely. Many operations in the GT6 are best done with one seat or the other removed, which takes very little time (4 bolts per seat) if the fasteners are maintained.
I don't know what the replacement harness you'll be getting will be like, but the only thought-provoking area is the harness where it connects to the fusebox. The wires in the back of the ignition switch are blade connectors after removing the cap in the back.
The key to keeping all the electrics under control & not getting frustrated with future troubleshooting etc. is cable ties. The original harness even though wrapped in branches is still like a rat's nest when installed due to long free lines. Get a package of small cable ties, & when each harness is in place, gather the wires after the wrap ends as they continue in reducing groups to each individual gauge, switch, & light with a tie every couple of inches. You will be surprised how much better it looks, & how it then becomes as simple to look at as you'd think it would be when looking at the wiring diagram.
The harness which runs from the dash area to the rear of the car (taillights, gas gauge sender, etc.) goes down the left side I think, then turns around behind the rear panel, travels across, & terminates at the rear sidemarker on the other side. You will need to remove the seat on that side anyway to get at that, & you will need to remove the two vinyl/leathercloth upholstery panels on the sides in the rear to run the harness & get at the connections for the rear sidemarkers. Use one of those slotted spatula-like tools to pop out the upholstery panels, or the force of pulling the clips out will tear the cardboard-y backing of the panels that the clips attach to.
I don't think you'll need to remove the black cardboard interior panel at the very back underneath the hatch, as the wiring in that area is accessed from under the spare tire & gas tank covers. Don't fool with that black panel ever if possible, it is VERY prone to disintegration & tearing. Nothing to see under there, move along.
More typical than problems with the wire itself are problems at the connections & with the units themselves like the taillight bulb holders, etc. You can clean up connectors with a small piece cut from a Scotchbrite pad, or with little wire wheels on a Dremel tool. There is one wire "wheel" that is like a Q-Tip & works very well for getting inside the metal loops where the ground wire 'bullets' connect.
On the taillight bulb holders, often there is discontinuity in the unit on the ground side of the holder. It's a difficult problem if you identify it, as the solution is either a big glob of solder applied with a soldering gun in a good location & lots of luck, or just replacing the holder with a new one.
To get to the wires going to the door switches, it will be neccessary to peel away the carpeting on the sides of the door jamb before the footwell. I cannot remember where the wire to the dome light runs. I'd say just leave that run if nothing's wrong with it.
When I removed & unwrapped the engine harness last summer, I was surprised that the wiring was in such good condition. The short wasn't in the harness, so I re-installed it after I re-wrapped it with the same-width wiring harness wrap tape from ebay (wiring harness wrap tape is not electrical tape, it does not have adhesive but it does cling to itself). Yours may be different, if a previous owner has added aftermarket stereos, aircraft landing lights, etc. with lots of evil Scotchlok tap doodads here & there.
Edit:
Where the wires pass through the sheetmetal there are grommets of course, some are orignally of sort of strange design (solid with pierced center, etc.) I'd suggest being sure you've got all-new grommets on hand before starting.
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