Just an FYI for anyone interested, because I couldn't find the information anywhere. If you're looking to install LED bulbs in the rear of a 964 that has cruise control, you'll need something like Tore's (wonderfully made) Smart Ballast. If you're curious, the wires needed to spice are the [Black] and [Brown&Blue] wires for your rear brake lights. My rear lights are newer production, 2016. I've had his LED retrofit kit, as well as the T-Lock system in my 1993 964 C4 for 3000+ miles with zero issues. Heavily recommend. For a daily driver, they're both a must.

all you need to do is leave the third brake light with the normal bulb. when I installed L E D bulbs I put one in the third brake light and got feed back through the system put the stock bulb back in the third brake light and every thing is great again even cruise control works

Same here. When I installed leds all around the PDAS stopped working. So I put a standard bulb in the 3rd brake light and the green PDAS dashboard light came on again.

If you mean the rear high mount brake light, I hadn't even touched that one (looking to upgrade it to a 993 RHM brake light in the future). My post was more to submit my feedback on the product, and offer information to anyone who might need it. I updated Tore on my findings as well. All my bulbs (save the RHM brake light) are retrofitted with LEDs and are much brighter than stock. And I haven't experience any feedback related to LEDs (just audio kind from the ancient speakers and 90's aftermarket head unit - present before the retrofit). What years are your 964's?