With the help of advrider.com “I” found the plug on the 300 L. This time is under the headlight on the right side. Same plug as on the 250 Rally so scoring a matching connector should be easy.
I’ve ordered the connector from 12oclocklabs -> http://shop.12oclocklabs.com/index.php?route=product/product&path=25&product_id=159
To get access you need to remove the headlight. It’s attached via 4 bolts 2 on each side. Notice that the top right one is different and in order to get the light off, you need to undo the bolts and pull/wiggle the whole thing right. There is a metal “spacer” inside the top right bracket that is fixed to the frame that the light attaches to, so the light does not pull straight out after you undo the bolts. It was a bit tight the first time I did it, but don’t brute force the thing.
The connector finally came from 12oclocklabs. I had previously bought a USB socket that I have had wired straight to the battery. Now that the connector came, I soldered the two together and mounted it back on the bars.
Not a very challenging installation so maybe instructions are not needed. In short: shorten wires, connect red to red, black to black, mount USB socket, plug in connector, reinstall headlight.