you most definitely have shorted wiring. The left turn signal is lighting on the dash because of stray voltage from the low headlight wiring. This draw also causes the headlight to dim as two bulds are being powered from the same source. Well, you could say 3 because of the dash light. The turn signal and headlights are completely independent circuits so the only explanation is they are shorted together. Were those halo's spliced correctly? also fish around and find their grounds as they may be common. This should be really easy to find...take out that headlight assy and check all the wiring. Something could have pinched the wires or cut into them.
So far so good! One thing that wasn't mentioned, the last time the system went crazy I noticed (just before it all started) that my voltage was rather high (around 15-16) First the ABS/INOP light, then the turn signal indicator, followed by the meth kit turning itself on at idle ... A few miles down the road the issue seemed to correct itself.
It wasn't untill the following morning that I popped the hood and found the cable to be kind of loose. I have put about 60 or so miles on the car since both terminals were cleaned and tightened and no issues! I have been told it may be a good idea to use a longer bolt on the cables since optimas seem to have an extra "lip" or platform that restricts the amount of threads that are usable. I dunno, keeping my fingers crossed!