Since both sides share the flasher unit, it is either in the bulbs or wiring. A common problem is corrosion in the bulb socket or corrosion where the socket plugs into the wiring harness. All it takes is a little corrosion, or a bad ground, to make this issue happen.
Put the 4 way flashers on and walk around the vehicle. Compare the lights on one side to the other. Look for a bulb that isn't lit or is dimmer than the other side. Repeat with the parking lights on as well.
Put the 4 way flashers on and walk around the vehicle. Compare the lights on one side to the other. Look for a bulb that isn't lit or is dimmer than the other side. Repeat with the parking lights on as well.