Assuming the wiring system is the same as that for an '00 M900, which may not be true...
Use a voltmeter and check to see if you get voltage at the Green/Red wire at the taillight when you actuate either brake lever. If you do, then either the little brass contact that bears against the nub on the taillight bulb isn't touching, or that bulb that looks good isn't. I've had bulbs that looked fine, but a filament would not light up.
If you don't get voltage, then check the Green/Red wire at each brake light switch, see if you get voltage there when you actuate each brake lever. If you don't the switch may be bad, or...
With the key on, you should get voltage at the brake switches on the Violet/Black wire.
If the horn works, the associated fuse, #5 I think, is OK. That fuse also controls the voltage to the brake light.