My '90 Taurus keeps blowing the fuse for the turn signal and the mechanic can't find the short. Here is some info:

the fuse blows only when the brakes and turn signal are on together;
the owner's manual says the brake lights and signal are on the same fuse but they don't seem to be because the brake lights work fine;
the mechanic checked the bulbs and spent a couple of hours looking for the short in the steering column, with no luck.
Any ideas before I take it to the really expensive electric system specialist?

You are going to have to physically check the wiring. Start at the tail lights and work your way forward. The insulation between the turn signal wires and the insulation for the brake light wires may be worn or burned off causing a short between the two circuits. Does it matter if the left or right turn signal is on?
Make an enlarged copy of the wiring schematic for your car and highlight the wiring for the brake lights and the wiring for the turn signals. Then check off the sections you have tested and inspected.
Be patient and methodical. Electrical problems can be time consuming to find. Start with the exposed or easy to get to locations in the wiring harness to see if you can isolate areas of the wiring harness. Make sure plugs (connection points for the harness) are not corroded or burned. Shorts can hide under the wiring harness wrapping. You already have a good idea that the problem will come after the brake light switch located near the brake pedal.
If you have to unwrap the harness in places, don't use adhesive electrical tape to rewrap the wires. Places like Home Depot sell non-adhesive electrical tape to wrap wiring harnesses. Just use a small piece of adhesive electrical tape to secure the ends of the non-adhesive electrical tape.

Have you installed a 3rd party illumination/light control switch, radio/entertainment system or navigation system?

Check to make sure the proper light bulb is the appropriate light socket fixture.

I would check the turn signal / wiper switch.

After all that fails get an in-line dc voltage checker (they make models that clip over existing wire so you don't have to splic anything) Check for voltage amount and amp leakage in various spots.

You have a good point there. Maybe if a 2057 bulb (dual filament) were forced in backwards, the contacts could be shorting across both terminals causing both filaments to attempt to light when the brake or turn signal is applied. Make sure the bulb is properly installed. They are tabbed such that installing the bulb backwards should be difficult, but with a plastic socket, not impossible.

-Rod