IN 2002 Buick LeSabre left turn rear light and left brake light don't work. Whenturn signal switch is turn left blinker light has with very high frequency.

the high frequency of blinks means you need to replace one or both bulbs on that side.

It sounds like you could have lost the ground wire connection on that corner. Check the continuity to ground with a meter if you have one. You can install a new ground wire from the black wire to a metal screw into the body of the car.

all bulbs are OK

when I click "lock doors' all lights are on, so that bulbs are OK

When you lock doors and lights come on that is that one bulb or two coming on? Replace the Brake / Turn light and that should take care of it. The full light assembly must be removed to get to bulbs. I had the same thing on my 02 LeSabre.

Thank you. But this is not the case. I looked attentively. All back rear lights were on (identically on the left and right sides; I don't remember : 3 or 4). An important factor you missed. When I tested left and right turn lights, frequency of blinker inside the car is about twice higher for the left side, which doesn't work (left rear light didn't work but the front one works).

Thanks. Without electric scheme it is difficult to determine which fuse controls the left turn and brake circuit. As to ground, usually there is only one ground wire. Since all other electric modes are OK, this is not the reason.

The rear bulb has two filaments. The one for thebreak/turn is burned out. The tail light one is good. Replace the bulb

The multifunction switch is a known failure part....try and get to the probably three plugs running down the steering column and clean or reseat for good connections. and see it that helps, if not may need replace it, that is what I am doing at this time with a 2004. BE VERY careful around the steering column and the air bag wires and connections so as not to set it off....

someone had put a 3156 bulb in and the internet called for 3056 or 3057 bulbs, The correct bulb is in the owners manual and is a dual element 3357. The wrong bulb blew the fuse in the fuse box under the back seat. The fuse panel is confusing. You need to replace the 10 amp fuse #13, which the mechanic and I both misread as the wrong (15amp) fuse. New bulb, new 10 amp fuse and all tail lights are working: tail, blink, break, corner, etc...