I gave it another try and succeeded to squeeze the connector trough frame. It was tight between the steering lock and frame tubes, but it goes when the ECU box is removed at first.
I rearranged the cable harnesses under the gas tank and placed the BEP on right side, covered carefully with rubber (against moisture and securing the connectors). OEM dashboard cable goes over the ECU box to the right side now. I tested all the electrics that can be tested without starting the engine (the tank is still on painshop).