What makes the lower left fault indicator light on the dashboard come on and remain on when I'm driving?

This could have something to do with the electrical system to the rear of the car. Make sure all the bulbs are good - even if they are all working they could still be the problem. Replace any bulbs that look smokey on the inside, especially the center brake light. The warning light can also be triggered by the wrong wattage bulb installed. If the bulbs are not causing the problem check the lamp failure relay on the left side in the trunk behind the carpeting. There have also been some frequent problems with wires breaking in the trunk wire harness on the left hinge to the body. The wires stretch and break inside the harness and it may be necessary to open the harness and repair any broken wires. Good luck!

Yellow box next to hinge 89373-33010 Also look at wire harness at hinge cut shorted