I've tried a couple of different LEDs in the tail/brake lights, no issues with ABS or CC, but the high level brake light did flicker faintly.
Looks like those 'classic car LEDs' have the same problem as chinabay cheapies - the 5w circuit being far too bright - so it's harder for following drivers to decipher whether you've pressed the brakes or not.
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MY97 940 CD LPT Super Sports Edition 2 Turbo, Midnight Purple, 175,000 miles.