Improper lamp operation is almost always due to a bad ground at that light assembly. Current goes through the intended bulb, can't complete its path to ground, and it finds an alternate path through another bulb, then to ground on that mating bulb on the other side of the vehicle. In this case, the turn signal and running light are in the same bulb and the two filaments share a common ground connection. Current goes through the signal lamp on the left side, then backward through the running light, then over to the right side, through that running light, then to ground over there. You can prove that by removing the right signal / running light bulb. That will make the left bulb not work at all.
Without looking at a wiring diagram, it may also be possible for current to get to ground through the dash lights instead of the right running light. The dash lights are connected to the running light circuit. When this problem occurs, the voltage is divided up between multiple bulbs, so the dash lights and / or the right running light will be not nearly as bright as normal.
Oct 27, 2015 at 5:24 PM
