Also you can check the wiring itself. The wiring in the trunk might have gotten damaged by a sharp trunk object. You can pull back the stiff black liner fabric to see the wiring underneath as it goes into the rear lights.
It could also just be slightly disconnected and making sparks. You can look to make sure the plastic connectors are snugly connected to each other, and then also check to make sure the ground wire is securely fastened - there is a nut that secures the ground line to a bolt inside the trunk, which could be loose too.
Oh, also, you can see if the problem is from the front or the back. Do all four turn signal bulbs burn out? Or is it just the two fronts, or the two backs, or the two on the left side or the two on the right side?