Led rear light

[eviljim]

I needed to replace my rear light and plate mount. I think the one on there was aftermarket. So the light was fine but I kept on smashing the licence plate, I think the wheel was hitting it.
I bought a cheep unit of eBay, knowing it was a led and ment for DC, thought I make up an inverter if the flash was to bad. Anyway quickly trying it before mounting it seemd ok, flash was not to notable. So I put in on and the connect it properly, and it acts very strangely. Main light is ok. Brake light works at low rpm but not when reved. When light is on then the brake light does work.

Any ideas? I ran out of time so did not get a multi meter on it. I don't have a CRO.