I have battery disconnected and ECUS out. I want to wire an extra brake light in aND test it. Can I do so with ECUS out of the loop.? Thanks

I've done it and lived to tell about it.  The bike will power up but an error message will be displayed on the screen but as best I can tell no damage will come about as a result.  Obviously the bike will not start or run but the electrics are active.