Call66GT has a very good point. When working on cars, it's 99.9% sure that whatever you did to it last is what caused the problem, even if it seems unrelated. So, have you done ANY work at all to it? If so focus there.
If not, then check the easy stuff first.
Pull the tail light lens and check the bulbs. If one is heavily corroded it could be causing a short. Also, check your trunk wiring for any fraying, especially as the wiring harness disappears into the wheel well. They can get caught on sharp metal edges there and fray and short.
Next would be your flasher unit. It's the small silver cylinder with 2 wires going into it, tucked up under your dash near the steering column. Again, cheap fix and easy to check.
Next would be your turn signal cancel cam. This is the white (or dirty white mostly likely by now) plastic part under your steering wheel, at the top of the steering column. This is where your turn signal lever goes into the column. You'll need a steering wheel puller to get the wheel off. Then check for blackened areas, loose connections or melted plastic. This area sometimes gets hot if there's a short and will give intermittent problems.
Good luck. Post back when you find it.