Volvodad is on exactly the correct track, though the readers might be helped by some clarification of the bulb failure circuits. This system relies on a certain level of balance between pairs of filaments on the three circuits that it monitors. If there is an imbalance of currents between the two bulbs, then the alarm switch for that circuit is closed causing the warning light to come on. Interestingly enough, if both filaments are open, then there is also a current balance (both zero) and so the warning light does not come on.

The problem described most certainly originates from a ground path that is shared between the turn signals and tail light on the left rear. A sloppy ground simply results in a slightly increased voltage at the ground terminal of the light. It should be nearly zero, but let’s assume that it has increased to one volt. Now, the voltage supplied across the filament is down by a volt and so the current will be low, too. This causes an imbalance, but not to the level needed to cause a warning in the BFS. There must normally be at least a little margin to eliminate false failure reports from the normal mismatch between bulbs. This might be on the order of 10% to 15%. Actually, this is why you should always replace these bulbs at the same time, and with good quality, matched bulbs.

Let’s now assume that the turn signals are on. The added current, nearly three times that of just the tail light, will cause an even greater ground loss due to the increased current through the sloppy ground. Now, it may be 2.7 volts. In this case, the imbalance is substantially worse and has crossed the threshold of sensitivity and now causes the switch to close and the warning light to come on.

There are three connections to consider when trying to rectify this type of problem: the screw where the ground wire attaches to the body, the other end of the wire on the taillight assembly connector socket, and the bulb contact to the socket or PCB. I’m uncertain what the taillight assembly looks like on this model, but the problem will definitely be in the area.