Most likely they share the same bulb and you have a bulb burnt out.
Purchase the correct replacement bulb(s) first, before disassembling the vehicle. See sources and related links below for replacement bulb information and full light bulb part numbers list. Then consult your owner's manual for the headlight bulb lamp replacement procedure. As simple as it sounds, the owner's manual is the best place to start for bulb replacement instructions and illustrations. You may want to consider replacing both left and right at the same time in order to keep brightness and color equal (optional).
the bulb with two elements in it is the turn signal and brake light bulb
If only the right rear brake/signal light is effected it is not the fuse. Inspect the right rear brake/signal bulb, socket and wiring.
the turn signal switch is probably bad
More than likely you have a burnt out bulb at the front signal light, rear signal light, or brake light on the right side.
Usually when ones turn signal is blinking fast its telling you that a turn signal bulb (and in some cases a brake light bulb) is out. If it blinks fast when you turn on the the left signal the burnt out bulb is on that side of the vehicle (could be front or rear bulb so you'll have to check). And vice versa
Check your bulb, In the rear of the Generation 4 Ford Taurus, they use a single Dual filament bulb for the marker, turn signal, and brake functions. Chances are your bulb is burnt out and needs replacement.
The turn signal and brake light use the same bulb. When you get a fast flash, it is from a reduced electrical load being placed on the flasher unit due to a burned bulb. Replace the bulb and this should fix your problem.
In many cases, the brake light and the turn signal use the same bulb.
You almost certainly have bulbs burnt out. Answer 2: Check the fuses, as well as the bulbs. If all that is good AND your system uses the same light (Brake Light) for the turn signal, then check the turn signal switch as well (the brake light lead passes through the turn signal switch). If that is all good, or it is a different system, then check the wire leads running into the rear section; most have plugs that could have become disconnected.
Bulb is out.
Start with checking the left rear brake light bulb.
short circuits in the turn signal and brake light circuits