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Think this through methodically: 1. Something is providing 12 volts to the trailer. Unless there's a battery on the trailer itself, that power must be coming from your tow vehicle. (You don't say if it's some sort of travel trailer than has an onboard battery.) 2. Somehow that 12 volts is finding its way to the right brake light, which is presumably the right turn signal as well. But it is NOT turning on the tow vehicle's right turn signal. 3. Interrupting the connection between the truck and trailer stops the current through the right turn signal. It sounds like you have multiple problems that are combining to cause your problem. If the ground for the right turn signal on the trailer is bad, any voltage on that bad ground will feed back through the turn signal and find a ground via the vehicle's turn signals. Actually, multiple turn signal lightss are energized, but only the trailer light has enough current going through it to light up, It could be that there is a relay that is being energized by itself, because of a short circuit. Disconnecting the trailer deprives the relay of voltage so it de-energizes until the right combinations of events happens to re-energize it. First thing to do is to find where the voltage is coming from. Use a test light, or better yet a voltmeter to determine if any of the trailer plug connections are hot. That will give you the source of the voltage. Then you need to determine if the vehicle or trailer is at fault.
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