I'd like to add an LED brake light which is activated by deceleration. I have the light, now I need to connect it. I had a look at stock taillight and it appears to have a connector for add-on lights. Does anyone know where I could get this three-pin connector from or what is the type of this connector? Indeed, I could just solder it to the existing wiring, but plugging it into connector is more elegant solution.

 

Interesting. No help here with the wiring, but I would like to know how you are gonna switch this set-up to keep the light from coming on during gear changes.

 

There is no deceleration when shifting up. So the light will not come on. When switching down and actually decelerating the light will come on and flash. This is the idea of this light, to warn drivers behind me I'm slowing down. I'm not sure I understood your question ...

 

Any non-standard lighting or flashing creates ambiguity in the minds of other drivers. "What the hell is that guy doing?" Just say no to lighting schemes that rake not DOT certified. Your idea is specifically illegal in many states.

 

So when I use brakes to slow down then brake light coming on is OK, but when I slow down using engine braking then brake light is illegal?

 

Where is this deceleration, slowing speed signal coming from?

 

Mercury switch, built into LED light, adjustable - to make sure it won't give false positives.
But really, I'd like to have an answer to my question instead of discussing how illegal my setup might be. TBH I just want an extra layer of security on braking. This LED light is very bright and it will definitely come on when I use conventional brakes. It will not come on when I slow down on a high gear. I see no issue when it comes on when I switch down to second from 60 MPH to slow down in wet, for instance.
You do know there are some benefits using engine braking instead of rear brake, don't you? Engine brake will never lock up your rear, so you will never lose the grip in rear. I used to ride in snow and ice when I was young and the engine brake is the lifesaver. I still use it alot, although snow and ice are extremely uncommon here in southern LA.