This is characteristic of a bad ground wire at the rear. You can identify a clue by turning on the left signal light, then observing the right side will glow slightly. Sometimes that can be too hard to see, so another way to verify this is to remove the bulbs from the right side. If the ground is bad, the left side will stop flashing or it will flash differently. If you turn on the brake lights, you may see the front signal lights glowing dimly. With the rear ground broken, current for the rear lights finds an alternate path through the interconnected front lights, then to ground up there in the front.
Also check if the license lamps are working.
The ground in question is "G200" shown at the bottom of the second diagram. AllData online service manuals went down for service just as I was trying to copy the ground location diagram. Typically that will be a wire bolted to the body sheet metal or the frame near one of those lights. It could be bolted to the box behind a tail light housing to protect it from moisture, or it might be bolted to the bottom of the box.
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Wednesday, June 2nd, 2021 AT 1:23 PM