Well you can buy a "Haynes Auto Repair Manual" - that covers your year and model car.. Available at most auto supply stores in US, - Other countrys might require a little searching, but they are around! Last one I bought for a car here was under $20 There are complete wiring diagrams in back of book for each model!

Or you can use "reverse engineering" and pull all the fuses from fusebox, except for one that supplies power to intrerior of car,-- turn on key, and hunt fot "hot ends" (suggest that you also remove wire from ignition so you don't have "hot line to engine" - -- if you have points it (distributer) could burn them out while you are hunting! Note that wires could be "degraded' in color and so faded that you might not recognise it either! -- But using a "test light", or voltmeter (to ground on other end) will at least prove that the wire is on same circuit (pull fuse to confirm it goes dead)! .. And it should also go on and off with key! Has to be a pretty good sized wire, - At least a "12" gauge or bigger- as it carrys quite a few amps in use!

Third method is to find somebody graciouis enough to let you look under dashboard... unfortunately Triumph was real good at putting wires in impossible to see places,- unless
You have part of dash pulled, but possibly with a small mirror, and a flashlight, you might be able to follow wire back to wiring loom group from switch! Admitted, it is a "bugger" to get down and look at mirror to see!

Or lastly the "punt method" -- Get a 10 gauge wire, and run it from main "bus hookup"-- to heater switch,-- you can buy a "in line fuse connector for protection, and use same size fuse specified for that circuit,- and just run along main loom across to fusebox! .... As long as it goes on and off with key, "you are in business"... Not perfect, but reliable, (and effective)! Needs to go off with switch, so motor doesn't run down battery, - if you forget to tun off!

If you look at the print power pegs from the fuse block, then to the loom where 5 wires are solder together in the loom. If like mine (old one) it will be up under the dash inside the loom. If all else fails you can peg power from the brake light switch. I had to do that on my replacement harness. On my harness the joint was against the bulk head just under the dash. If the old wire is hidden with the other wires and is still there the untermaniated end could set you up for a short or worse a harness fire.