I was wondering about the ground at first too but I was thinking more than just the taillight would be bad if that were the problem.
IRRC (again) my ground wire failures were right in the tail lamp assembly which would explain why the turn sigs work but the tail light and brake light don't.
The wire can break right inside the insulation and the break is invisible. If you flex the wire along it's length it will typically bend sharply where a break is located.
Using a voltmeter for continuity test works some of the time, but you can have a high resistance in your circuit due to corrosion or ?? and the voltmeter doesn't draw enough current to detect this.
For this sort of thing a test light with a bulb similar to the one the offending circuit supplies current to will give the best result because the current drawn by the test lamp is similar the required current for the bulb in the bike.
BTW I use a dielectric grease that will prevent corrosion if you liberally coat all the contacts in the socket and connectors. The fact that it is dielectric prevents shorting between connections if you get a little enthusiastic putting it on.
Ask for DC 111 at an industrial supply place or Permatex dielectric grease at the autoparts place.