Might be different for later TR6, but earlier cars used the same holder as my Stags; which grounded through the clip and lamp housing. The single filament holders only had one wire, double filament two wires.
The original holders relied on an internal contact, between the steel clip and the brass bulb contact, that wasn't mechanically joined except by the plastic shell. The brass/steel joint is very apt to corrode, especially after the shell sags a bit due to heat and age. Mine literally had about a 1/16" gap between the two metal pieces, and needless to say, no ground to the bulb. Oddly enough, the car also had cut harness wires and added red wires, as the DPO had tried in vain to get the rear lamps to work without being grounded!
But the replacements I got from TRF did have a separate tab for ground, a much better design, IMO, even though I had to create a ground harness for them.