This may be a dumb question but, does the engine ground bolt onto the battery or somewhere on the frame?
From the factory there's a big lead from the frame to the negative terminal of the battery, and everything else treats the frame as ground.
The headlight won't come on until the engine starts (unless the blue-wire mod as been done)
The tail light and license plate illumination should come on with the ignition switch in the 'on' position. The ground for those lights tie in very quickly to the ground for the turn signals. I've seen the wiring for the rear lights get crushed between the fender and the bottom of the seat, so check to see if you've got a short or broken wires there, especially where the bullet connectors are.
Try connecting the rear turn signals one at a time. Chances are that only one side is causing your power outage issue. The turn signals should be short/current overload protected by the main 30A relay, which would have blown if there was a complete short to ground.
When you say you lose all power, you're not getting any dash lights (i.e. red oil indicator lamp/running lights in front turn signal housings) and you can't start the bike when the ignition is on? Or are you just losing power to the rear signals?
