[dsraven]

Hey evil, are you able to post a pic of the actual lights and the wiring going to them?
Can you explain how you are testing, what kinda tester and where do you connect the ground? Have you traced the wiring back from the lights, does each light have a tail light wire that goes to the front or is there a T slice somewhere near the back where the single tail light wire gets split and one goes left, the other right? Sometimes those are a problem because they dont get sealed up well and corrode so there is resistance. Led lights like to run in a narrow range of voltage so resistance can mess with them. Same for the grounds, a ground is actually an extension of the negative battery terminal but instead of running a ground wire to everything it relies on many connections that are unprotected from the elements because "it's just a ground" and wont arc like a positive wire would. if the connection to an actual good ground has resistance it messes with the led's available voltage just like a poor wire connection would. It's just that, unlike an old incandescent bulb that would simply run dim, an led simply wont light up. Like mentioned, the light housing or wire can have an awesome ground to the box but if the box has a poor connection to the frame, well you get the idea. It's why I suggest to use a test light with some amp draw. An old fashioned tester may light up dim which tells you there is a poor connection. An electronic tester will light up because it senses voltage but any kind of draw on that circuit and the circuit fails. Also, like mentioned earlier, the tail light circuit is independant of the signals so if there is a problem involving both circuits at the appliance, light, then it makes sense to remove the appliance from the circuit, test each circuit, then check the common ground.
Just some ideas to test if you are still having issues.