Whoa there Nellie!!! If you connect the two black together and GROUND them, then find the power wire going to the LED stoplamp and cut once. Attach the + line (the one you cut leading to the stoplamp) to the box +12v (red) in, then the 'out' wire from box (red) to the other wire you cut leading to the stoplamp. This puts the flashing box in parallel (you power the flashy box) then that box outputs the new signal to the actual stoplamp assembly. No matter how you ground the unit (even if the original ground to the LED assembly isn't cut into), the power line MUST be interrupted and only outputted to the lamp assembly. This will insure the lamp works properly. You also could cut both the hot and ground and install it that wag but this way (above) will work and you only have to cut one wire to insert the reds into the power line feeding the stoplamp. I got confused with the way you said that above George, and I am assuming the BK/BL wire your referencing is the actual color of the wire in everyones car be it LHD or RHD. I have found wires not labeled correctly as when they manufactured the harness they sometimes run out of a certain color/stripe.