When installing an electronic item that is either not stock and an upgrade, or is totally not part of the original TR6 ideal (high-powered stereo, fog lights, etc.) I am a big believer in adding on a totally independent power source that has nothing to do with the original wiring harness. That way, the danger of overloading your harness and creating Lucas smoke is eliminated. It is really simple to access your battery directly for power with a wire that can handle the appropriate load and if you have the need to utilize the ignition switch for on/off demand switching, you can put together your own relayed mini-harness for each item. I wired my fogs and my electric fan this way and never worry about burning wires. Dan Masters' book on Triumph electricals has been a godsend for helping me design my wires. It should have been called Triumph Wiring For Dummies. He also goes into detail on overhauling most OE electrical switches; this is important because so many of the aftermarket switches out there are pure junk, as discussed in another post. (I made the mistake of lending my book and have yet to get it back. Neither a lender nor a borrower be.)
Oh, and I once had a high powered stereo in my TR6 but couldn't hear it over the dual exhausts. Once is the key word here.