Buy a 12V, dual element lamp with wires for contacts. Straighten those wires and crimp them into 1/4" male terminals. Pull the fuse and push the bulb into it's place. If there's a short, the light will burn brightly but won't hurt anything.
Then you can go look at any and all of your turn signal lamps. If they are glowing dimly they're fine, if there's no light at all, that bulb is probably shorted. Which is what I think you have, a shorted bulb. This is where the filament open circuits and falls off, shorting out the contacts inside the lamp.