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Because of a bad ground wire, and or a burned out light bulb.
Check your bulbs and the ground wires for each one.
A big help will be a separate "test" ground wire to use for each.
The two most common reasons for a bad ground wire are 1) rust or corrosion, or 2) a car wreck poorly repaired.
God bless your efforts.
Power flows from the battery to the main fuse. Then it flows to the control coil and contact side of the headlight relay. When you turn on the headlights, the headlight switch provides ground to the control coil in the headlight relay and that operates the contacts. The contacts allow power to flow to the right and left hand headlight fuses. Power flows to the headlights. The headlight switch is what provides ground to the high and low beams on the headlights.
To diagnose, remove the headlight relay. Using a voltmeter, check for battery voltage on terminals 3 & 4 in the socket with the key in the RUN position. Then turn on the low beams. Test for good ground on terminal 1 in the socket. If you're not getting ground, then you've got a problem with either a bad headlight combo switch or a bad ground. If you get good ground on terminal 1, reinstall the relay, turn on the low beams and test for power in the headlight sockets. If you get battery voltage, that confirms you've got good fuses and wiring up to that point. Next, check for good low beam ground on the red/green wire in the bulb socket. If you're not getting good ground, it's either a bad combo switch or bad ground from the combo switch. Then turn on the high beams and check for good ground on the red/yellow wire in the bulb sockets.
It wouldn't hurt to clean the ID ground on the left side of the instrument panel, but based on everything you've said so far, this sounds like a bad combo switch
You have a bad ground on the left low beam. Using a test light connected to ground see what wire is positive with low beams on. Using a piece of electrical wire connect one end to ground and with the headlight connector connected to the headlight and headlights on stick the other end into back of the connector at the ground wire. If the light gets brighter, you can cut the original ground wire to the connector ( 2-3 inches from headlight connector) and run a new wire from ground and connect it to the wire at the connector. Hope this is helpful.
If it is truly a ground problem you should be able to do the following.
NOTE* Sometimes finding the root of a ground problem in a wiring harness can be very time consuming and also drive you nuts.
First you need to establish if it is indeed a ground problem. Check your fuses and make sure that none of them are bad or loose. Look at the back of the headlight switch and determine if the connection is solid. Do the same with each headlight. If you find a dirty connector then clean it gently and re-connect it. Try the headlights again... If the same thing happens continue.
IF YOU ARE NOT MECHANICALLY INCLINED, FIND SOMEONE WHO IS FOR THE NEXT PART!
On the headlight connector there are usually 3 wires. 1 low beam, 1 high beam, and 1 ground, Find the ground wire from the headlights and provide a solid temporary ground by adding an additional wire into the ground socket. DO NOT CUT EXISTING ELECTRICAL WIRES ON YOUR CAR!!! Get a length of Electrical wire and strip off about 1 inch of the coating from each end. Find which socket is the ground, and slip the new wire into the socket with the bulb. Take the other end of the wire and fasten it to a grounded area, "something made of steel, like the frame or part of the engine. NOT ALUMINUM...
Try the headlights again... If it works ok then you do have a ground problem. The options now are to either secure everything down "as is" and consider it fixed, or, take it to a shop and have them fix the ground the way it should be fixed.
This is only ONE way to diagnose a ground problem but NOT the only one.
If it does not work then it is something else other than a ground, like a switch.