I am not familiar with USA lighting laws. But if your left rear is on as a daylight running light and it also flashes brighter surely that must be correct. So if you check the bulb it should be a twin filament bulb and have three wires to the lamp. If that's is the case then the right rear should have the same bulb and you have a possible wiring fault. Unless it has a control unit that uses one filament for both jobs. In either case whatever set up the left side year uses so the right side should be the same. If that makes sense.