Start button is constantly making contact, or short in the start button circuit near the steering neck connectors.
The taillight doesn't come on until after the stater energizes the headlight relay inside the fuse box, so the engine must be running
When you say the ECM, I am prone to believe you are talking about the Ignition Control Box [ICB]. If the ICB were o fail, it woulnd't cause the starter solenoid to be engaged with the key on.
Little more digging found these issues.
1. Power always on to the low voltage feeding solenoid when key is turned on.
2. No power to tail light (as should be there if the key is switched on) but light does come on when I press the "Start" button.
And again, if the switch is turned on, the engine immediately starts to turn over. So something is switched.
I think the ECM may be bad, but would like to test it prior to ordering a replacement. I can't find directions for test the unit.
first thing I would do is take a part the Starter switch, and with the switch apart try turning on the ignition (ready to shut it right back to the off position if the engine starts to crank). If the engine doesn't crank go to step 1: If not finish step A:
A: The starter button is a very simple switch to take a part and clean. just unbolt the switch housing from the handlebars and its pretty self explanatory to get to the stater button itself. Once you get to the starter button, pull out the little spring and clean it with brake cleaner, then apply liberal amounts of graphite to the spring where it resides inside the plastic button itself. on the contact side of the spring dab on a small amount of dielectric grease. look for carbon scoring on the switch contacts and use a piece of steel wool to shine the copper back to a shiny copper color.
Reasemble the switch and housing... retest.. If bike still tries to start after turning on the ignition switch with out the use of the starter button, move on to step 1:
1: Check the connectors under the clutch side neck cover for looseness, corrosion, broken connector, and/or possible shorts to ground. Unplug the 6 pin connector and test for power on the Blackd and Red wires (main harness side of the connector). If you have power on either wire, then you have a faulty Starter relay inside the Fuse (junction) Box and needs to be fix/replaced.
If no power present, every thing this far is good. Reconnect the 6 pin connector and go to step 2.
2: check the power coming out of the Fuse Box on the large white wire with the ignition switch off. (should be power). next check for power coming out of the ignition switch on the Yellow wire (should not have power). If you have power on the last test, you have a faulty Fuse Box and have to repair or replace the fuse box.