> Wow, if you
> depress the brake with the left turn signal on the back up
> lights come on and everything else is dark.
Sounds like a classic bad ground; but I think you have multiple problems
going on, and some of them may be in the cockpit.
I would start by getting a good, solid ground reference at the rear (run a
wire from the battery negative post if you have to), then blocking the brake
pedal down, turning on the left turn signal, and checking exactly what is
happening right at the bulb. Best I can tell from my Haynes, the bulbs are
all mounted in the same assembly, which is supposed to be grounded. If the
assembly isn't grounded, the current may flow backwards through the reverse
lamp and to ground in the other assembly.
Looks like the bulb holders ground to the assembly with just a spring clip,
too; which might not be making good contact. Dunno about TR7 bulb holders,
but the holders used on TR6 and Stag have lousy ground connections inside
the holder, too. Every single one was bad on the first Stag. The other Stag
has already had them replaced at least once, and still had two that didn't
This list supported in part by the Vintage Triumph Register
Triumphs mailing list