Tom,

One thing that I've noticed over the last few years is that manufacturers are fitting brake light clusters that have a higher output - probably better reflectors and better quality lens design. The problem for us is that we get used to this and it gets easier to miss the lower output from the older brake light design when driving behind an older car. There's no doubting the benefit of a third brake light but it might be worth going an extra step and fitting the LED stop/tail bulbs that are now available as well - they're a bit more expensive but they light fractionally faster and are brighter too. The total current draw would be much lower than the standard bulbs.

Aidan
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1967 131, 1969 131