Left headlight low beam went ouy, replaced it and still did not work. I figure it's the wiring But my shop manual headlight cicuit isn't shown.

If you give us year--make--model, I'll try to look up wiring diagram?

Check for power with a test light with the light connected. If you have power then its the ground. If no power then you will need to trace the wire back to find break. As I recall they did have seperate fuse for left and right headlight high & low beam. Also did you get a defective new bulb?

Check your connections at the wiring harness going to headlights for corrosion, sounds like high resistance somewhere, also make sure your ground wires are secure. It wouldn't be the alternator or the battery would be dead and not start the engine. Also shouldn't be any condensation in your head light assembly's or will cause bulbs to blow prematurely.

It could be your light switch is bad.

REPLACE MULTI SWITCH ON COLUM (STEERING) ON LEFT SIDE. bUY NEW ONE FIRST AND LOOK AT OLD AND SIMPLER TO CHANGE

Power for the headlights on the Left side (High and low beam.) comes from the same fuse... The ORANGE wire and the ORANGE/BLACK are the same, they connect in the harness. If you only lack the 12v to make the light work connect them at the headlight assembly.

If your low beam also lacks ground the Left low beam ground is shared with the right low beam ground... Run a wire from left low beam DARK BLUE to right low beam DARK BLUE/WHITE.

If its not working now then your cursed, seek a witch doctor.

take one of the bulbs ouy and closely inspect it, making sure the all filaments are connected across. if not replace them.....if the bulbs are ok it is definetly the dimmer sw. try using manual operation to be positive....

Check the fuse. There are seperate fuses for right and left. You dont mention if the high beam works for that side? If it does, the fuse is good. The GREEN wire at the bulb is the ground for the low beam. If that wire has power, then the ground is bad. If the high beam works, the hi'lo beam switch is good, and there is a problem between that headlight and the Engine Compartment Junction Block

Hi,
The simple answer may be that the low beam side is burnt out. Replace the bulb.
However, if both headlights have stopped working when switching to low beams, there may be a problem with the relay or the switch itself. If the cost of a relay is low for your car, it may be less expensive to simply replace that component than to have it diagnosed in a repair shop. Replacement of the switch requires some disassembly of the steering column or dash in most vehicles.
This is how to fix the low beam..
1.Check your fuses. The plastic panel covering the fuses should have a diagram detailing what each fuse covers. If yours is missing, or unlabeled, check your owner's manual, or a Hanes(tm) or Chilton(tm) repair manual. Often if a headlight is not working, it is the result of a blown fuse. Fuses have a wire running through them that will melt when too much current is run through them. If the fuse is blown, the wire will be broken. If the fuse for the headlight you are working on has an intact wire, it is likely the problem lies elsewhere.
2.Swap the headlight for its twin. If your left headlight does not work, put the light from your right headlight in its place. If it works with the other headlight in place, your headlight bulb is dead, and needs to be replaced.
3.If this does not work, try using a voltmeter on the plug for the bad headlight. Turn on the ignition, and turn the headlight switch. If the voltmeter shows no current, then either your switch or the wiring leading to the headlight has gone bad.
4.If replacing your headlight switch does not solve the problem, then there is a break in the wiring leading to the headlight, and the wiring will need to be replaced.
Thanks for contacting fixya.com

Ill guarantee you have an electrical short on the left side of the headlight switch,circuit.From here its immpossible for me to go any further.Good Luck.,Get A Wiring diagram.