Do it yourself, remove and change the headlight assembly on your Mercury Sable, the model produced between the years 2000 and 2005. You will require a flat blade screwdriver, an 8 mm socket wrench and a Phillips screwdriver. The whole procedure might take you about half an hour.
First of all, you have to open the hood and go above the headlight. You need to pull up and remove the two metal tabs that secure the assembly and then the 8 mm bolt, all seen in the pictures below. The tabs can be pulled up by hand, or using a flat blade screwdriver and the bolt using an appropriate wrench.
By doing that, you can pull the headlight out from the body of car. To remove it completely, you just have to unplug the main harness, seen in the image below. It powers up all the harnesses that reach the bulbs.
At this point, you can work to transfer the harnesses to the new headlight assembly, if required. You will need a Phillips screwdriver to do it, because the wires are secured by two small screws (see next picture). The parking light bulb can be accessed by twisting its socket counterclockwise and the main light bulb, by twisting the plastic ring in the same direction and then unplugging its connector and then by twisting a second plastic ring counterclockwise.
After you make all the needed replacements, install the new headlight into the body of the car and secure it using the metal tabs and the 8 mm bolt. This is all you have to do, if you plan to remove and replace the headlight assembly on your Mercury Sable, the model made between 2000 and 2005.