* The flasher unit itself: This is the most common cause. It's a small, usually black box, and its location varies by year and model, but it's often under the dash, near the steering column, or in the fuse box. Your owner's manual should have a diagram.
* A blown fuse related to the turn signals: While there isn't a "flasher fuse," there are fuses protecting the turn signal circuits. Check your fuse box (usually under the hood or inside, often under the dash) and look for fuses related to turn signals, hazards, or parking lights. Your owner's manual will show you which fuses are which.
* A faulty turn signal switch: The stalk on the steering column that controls the turn signals could be malfunctioning.
* Wiring issues: A broken wire somewhere in the turn signal circuit is also a possibility.
To find the likely location of the flasher unit and the fuse box diagrams: You'll need your owner's manual. If you don't have it, you can likely find a PDF version online by searching for "1986 Cadillac Sedan DeVille owner's manual." The manual will have diagrams showing the location of the fuse box and may even show the flasher unit's location.
In short, focus your search on the fuse box and the area around the steering column to find the likely culprits – not a "flasher fuse" itself.
