I want to have my 3rd brake light (located below the rear window) flash briefly when I apply the brakes. I got this (see link below) product from Amazon.com and installed it on my wife's Volvo- perfect, exactly the flash periods I want.

However, when installed on my 997.1 3rd brake light located below the rear window (I have a Misha wing, so my spoiler was disabled), the brake light flashes, but it throws an error on the dash when the brake is applied.

How does my car detect this "change"? Is it somehow measuring current or voltage? Am I assuming the voltage to the LED strip is DC (and the car uses some sort of modulation)?

Thanks for your help!

https://www.amazon.com/Purishion-Strobe-Controller-Flasher-Module/dp/B01HPSZZXQ/ref=sr_1_4?keywords=3rd+brake+light+flasher&qid=15 78336190&sr=8-4

 

I've used similar generic brake light on my motorcycles and they work very well there.

997 cars have a CAN Bus system that does a lot of "health checks" in the car. Typically, if your car is capable of bulb-out warnings, then you have a CAN Bus system.
This is the reason why when you switch halogen/incandescent lights to Xenon/LED, you will get bulb-out warnings (the system detects the drop in the load).

In your case, the system sees the brake pedal pressed, but doesn't see a load from the brake light during the off window of the flashing sequence.
So it thinks the bulb is out and immediately throws an error message.

There are CAN Bus compatible flasher modules, but are expensive.
For LED lights etc, since the load seen is much lesser than the stock halogen/incandescent lamp, adding an appropriate resistor to compensate works well.
But since the 3rd brake light is already an LED, I don't know if adding a resistor would help - as the system would see a much higher load than expected.

Ideally you need the resistor load to be activated only when the flasher relay is in the off window - which itself is like a relay logic
At that point, I would just pay more for a CAN Bus compatible brake light modulator (there are compatible headlight modulators already available, so brake light ones also should be there).

 

Even better, leave it as it was so everyone behind you doesn't have to see a flashing light show every time you initially step on the brake pedal. On a motorcycle where the brake lights are tiny, it can make sense but on a car that already has 3 brake lights it just adds unnecessary visual noise and I doubt helps to prevent an accident anyway.

 

As the OP, I ditched the flasher, and bought a 987 third brake light and mounted at the top of the rear window. It threw warnings, until I measured the current draw of the original LED third brake bar (about .17A at 12V), and the current draw of both the flasher (.21A) and the 987 third brake light (.22A). I installed a 9.5 ohm 5 watt resistor in series and the warnings went away.