Is a mercury bulb also called a driver? and where can I get one ?

Sounds more like a bad bulb socket. The bulbs provide a load which causes them to blink slowly. If just one bulb becomes disconnected the load changes and so does the flash rate. I would suspect that you have a bulb that is coming loose in the socket periodically thereby changing the load. Many vehicles us multiple bulbs in the rear...ex. my 04 grand am does the exact same thing. It uses 2 bulbs per tail light. One of the bulb sockets on the drivers side sometimes loses connection and they flash really fast (the other bulb flashes...just 1 of the two quits). In my car i need to replace the tail light assembly.

your flasher is bad its a round silver plug near or in fuse box

someone has switched the wiring harness sockets to the wrong bulbs or installed the bulbs in reverse. get in behind the headlight assembly on the driver side and reverse the wires, if that does not work then pull the bulbs and swap them inside to outside and vice versa.

This will sound stupid but check to make sure all the bulbs, yes I said bulbs, as in brake and turn signals are the correct bulb. No kidding. I fought a problem like this in my shop for a week on a mustang. It had a wrong brake light bulb. Just a idea. See if it will start by jumping the 2 posts on the starter solenoid under the hood on the inner fender. It has two battery cables going to it. use 2 heavy screw drivers to do this. Do not tough the metal of teh screw drivers. The solenoid could also be bad.

No you don't have to remove battery.
Look on the back side of the headlamp assembly and you will see to black retainer clips.Pull up on them and this releases the headlamp assembly,pull it forward then replace bulb reinstall the assembly and then push the retainer clips back down.

If the driver's side tail light is working, it's not a fuse. Use a test light to check for power at the bulb holder. One terminal will be for tail light, the other for brake/turn light. If the is constant power for tail light, replace bulb. If no power, trace wiring from bulb holder to driver's side junction. Probable broken wire between the two tail lights.

This problem is now resolved, thanks to a prior post by sableowner.

http://www.fixya.com/cars/t1479995-2001_merc_sable_headlight_replacement

The 2003 Sable has the same headlight assembly and removal/replacement instructions as the 2001 Sable. Once I had this information in hand, bulb replacement was easy.

i think the lamp cover just pops off. not sure though. assume you mean puddle lamp

You can look up the manuall on the internet or call a ford dealer and ask for fuse location for the signal bulbs

This is easy. Lift the lid open. Look at the back of the sealed beam and you will see a clip and an o ring. Pull the clip up and pull wires back toward the driver's seat. Unscrew the o ring and firmly pull the bulb out toward the driver again. Note the 3 tabs on the bulb seat and realign the new bulb with those tab ( 2 bottom, 1 top). Replace with #9007 bulbs..do them both.