My chevy lumina99 has no power on the back left side all the lights and signals work just that back left is my problem

Circuit/System Description
Ground is applied at all times at G201 to the turn signal/multifunction switch. The turn signal lamps may only be activated with the ignition switch in the ON or START position. When the turn signal/multifunction switch is placed in either the TURN RIGHT or TURN LEFT position, ground is applied to the body control module (BCM) through either the right turn or left turn signal switch signal circuit. The BCM then applies a pulsating voltage to the front and rear turn signal lamps through their respective control circuits.
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Did you check the RT STOP TRN fuse 15amp insturment panel fuse box.
Fuse Block - I/P
Left of the I/P behind the side trim panel
If the fuse is good , you could have a wiring problem or the BCM is bad .

Has the vehicle been in an accident on the left rear side. First check the bulb to be sure it is not defective, trade it out with the side that works properly to test. If the bulb works well then My thought are you may have a shorted wire. The turn signal flasher works by heat build up within the flasher the heat is caused by the load of the bulbs, the load heats the metal spring causing it to open the circuit then as quickly as it cools down the circuit closes again lighting the bulbs.
There is also a possibility of a defect in the turn signal mechanism on the steering column. The turn signal switch on many models does three different things on many models that use the same bulb element / or bulb for brake and turn on rear of vehicle. It controls the circuit for brake light by sending power both left and right side rear when turn signal is in neutral position then when turn signal is in left turn the switch cancels brake light power to left side rear and supplies flasher power to left side while maintaining brake light supply to the right side rear, and so on opposite when the right turn is chosen on turn signal switch. The turn signal indicators on the dash normally run off the same circuit as front turn signal bulb only which is also controlled by turn signal switch. Front has turn signals but no brake light.
I think there is an additional load on the left side rear turn circuit and it could be either a wire that is shorted or the turn signal switch if the same bulb element is used for both functions.
Often times if trailer lighting is connected the flasher will also blink a little faster because of the extra load of additional bulbs.

have you checked the bulbs front and back. to see if they work, bulb check. then it sounds like you might have a bad signal flasher that only works the left side and not the right side. or if a bulb at the front or the rear gets bright but not the opposite bulb on the same side then you may just have a bad bulb or socket or power to the socket when doing the bulb check.

I don't believe it would be a bad ground for just one light. It could be a wire, a socket connector, or the bulb socket itself. See if the wire has power at the socket when signaling left, with key on. If there is power, probably a bad socket or connector. If no power, check for power coming from flasher-the wire going to front left, or driver's side. If you have power from the flasher, but nothing at the socket, the wire has probably broken.
Good luck.

Fast flashing dash signals mean there is a bulb issue-DEFINATELY!

It could be a bad multifunction (turn signal) switch. The way that switch works, it is possible that it can turn on the front blinker and not the rear one. It also cancels the brake circuit so the turn signal can work with the brake pedal depressed.

However, a more likely cause would be a break in the YELLOW/BROWN wire at the rear of the truck. It is common for the connector to get corroded at the left rear frame area where the bed harness connects to the vehicle harness. If you have trailer connections spliced into the wiring harness, these connections are also prime candidates for corrosion. I would look there first. Inspect these harnesses thoroughly all the way up and into the tail light assembly.

If the wires under the back of the truck look good, then I would check for power coming out of the turn signal switch. The wire at the turn signal switch is plain YELLOW (with no brown tracer). It is Pin (M) on the turn signal switch connector. The front signal is a LIGHT BLUE wire at the turn signal switch Pin (H). If you probe these with a test lamp, they should both work the same except the yellow wire will be hot with the brake pedal depressed and the switch NOT in the left turn position. If there is no power at the switch connector, replace the switch.

you need to check the "combination turn signal/hazard flasher/daytime running lights module"
it is located in the junction block,the junction block is adjacent to and left of the steering column,you need to remove the lower dash panel below the steering column,then remove the steel knee brace.

check fuse.if good you got grounding problems or faulty turn signal switch.

You need to check for power & ground at left rear socket, and clean it & contacts in it well, and may have to squeeze socket a bit to supply better ground to outside of bulb base. If that doesn't work, poke wires going into back of socket with test light, with signal on, and if power present, replace the socket assy. About $5.00

the problen is probably the turn signal switch with a test light check for power at brake switch if no power check for power at turn signal switch the brake lights are incorperated through the turn signal switch