Just wanted to bring some visibility to this issue - it's documented in few places but not super centralized.
I reticently replaced all of the incandescent bulbs and fixtures in my 2001 S with LED bulbs. I also performed the daytime running light hack so my fog lights are always on. As a part of this, I replaced my side markers as well as my third brake light with new clear units. The total cost was about $200. I bought the bulbs on Amazon, and the new side markers and brake light on ebay. This is an easy upgrade that very much modernizes the look of your boxster - I'm very happy with it.
The LED headlights, foglights and brights are vastly brighter and a much more modern color temperature than the old incandescent builbs. a massive upgrade. The only downside is the led DRLs (the foglights) are causing interference for my AM radio - I will eventually fix - I probably need to swap out for a different brand of LED bulbs.
The LED side markers, turn signals, backup lights, and brake lights are also a huge upgrade. Much brighter and much better color temperature - the reds are more red and the yellows are more yellow. No downsides here at all.
A few months after completing this project, I noticed my cruise control wasn't working. Because I've done so much work on the boxster in the last six months, I wasn't sure exactly what I did to break it - in addition to the items above, I had the transmission out, and also I enabled the on-board computer and installed a new stalk on the steering wheel - I suspected first that this was the problem and either I had forgotten to reattach the lead to the cruise control stalk, or my mechanic had turned off cruise control when he turned on the computer when reprogramming the boxster.
Thankfully I did some research on-line before I started debugging. It turns out that upgrading to all LED brake bulbs and fixtures is well known to disable the cruise control on a 986 - some sort of safety protection system for the cruise control is triggered when the boxster sees too high of resistance across the brake light circuit; the computer apparently thinks that your brake lights are all burned out as it is expecting some current to leak across incandescent bulb filaments, which doesn't happen with LED lighting.
An easy solution is to solder in a 6 Ohm, 50 Watt resistor in parallel across the power feed into the third brake light. I purchased a resistor kit on Amazon, and then screwed it to the back of the rear firewall, inside the rear trunk and behind the trunk lining. Then I soldered one lead to the positive power lead going into the third brake light fixture, and I attached a lug to the other lead of the resistor, and grounded this at the rear firewall.
Here is the resistor I purchased.
https://www.amazon.com/gp/product/B00L4V9ECY/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1
Cruise control was immediately re-enabled once the resistor was installed.
When you do this, it's important to firmly attach the resistor to a metal surface in your boxster to help it dissipate heat, and also to make sure it's not touching anything meltable or flammable - it can get pretty hot as it sinks current (it's heating up kind of like a Halogen bulb) when your brake lights are turned on.
I read that in the distant past, someone was manufacturing and selling a more sophisticated device that looks like a halogen bulb to the boxster computer when it is testing the circuit at startup, but then disables itself when the brake lights actually go on, so you don't have a heat problem - but I couldn't find one of these available now that we are in 2021. If someone has a lead on these, please post here as they are much more elegant than a simple resistor and they won't create lots of heat like a resistor will.
Linear Mode
