BUT . . .
If your behind-the-dash wiring is, 1)original, or worse; 2) it's been attacked and defeated by a DPO (most likely), it is in desperate need of rewiring.
Under dash rewiring is a tremendous safety upgrade, IMHO. Many, many electrical fires have their origin under the dash area.
Hard to accomplish? Umm, well, maybe so. Depends on the condition of your car, and your skill level, as always.
Label everything as you remove it; get the right tools for the job (a good wire splicer/cutter, etc); solder what you can; use modern connectors (unless you're concerned with originality, then the bullet types are okay); use appropriate wiring gauge for the job and NEW wire; use dielectric grease on connectors. Folks like British Wiring Specialists can provide the correct stuff, if you're interested. It's not terribly expensive and well worth the price.
While you're at it, install (and hide) a modern, blade-type fuse box and use modern fuses. You can keep the old box for the sake of originality, if needed. I suggest to put relays on both headlamp circuits, and on auxillary lamps as well. Very big safety issue, IMHO. Use Bosch relays, they're cheap as dirt and easy to hide.
Under dash rewiring is the second thing I do to an old car, right after rehabbing the brake system. Both are critically important for safety and peace of mind as we howl around those twisties at 60 MPH!
It's not a terribly difficult project. Just accept the fact that it's intricate work, and set aside plenty of time to accomplish it (this reduces stress, I've found).
Then plan ahead and block out the appropriate time required; buy and get all your stuff organized, and don't get in a hurry.
Good luck!