the only other thingI can think to try to prove the loom is duff is this, unplug the rear brake switch and secure a piece of wire (jumper) between the brn connector to the loom and the battery +, you should find that lights, horn etc now work...disconnect the keyswitch and using your multimeter check the voltage at the loom brn again, you should find you have nothing at that point if your loom is broken. Take the utmost care with the jumper connected to the battery because if you touch any frame components it will spark, so it needs a lot of care. Only do this if you feel competent not because I told you to I cannot be held responsible for any damage caused by your errors. It would be safer with an inline fuse in the jumper too.