It's usually the front bulb or connector that causes this, since you said it's only on the left side. If the bulb got replaced with the wrong one, it could be a different resistance. Try removing & reinstalling the bulb, to tighten the connection, then just put a new bulb in if need be.
Did you check out your reverse lights? I've had this many times and 9/10 it's due to a faulty or failing light bulb. I'd go through and replace each set of bulbs until the problem is found. Doesn't sound like a relay problem here. Good luck!