How do I fix flashing head lights on 03 galant gtz?

If they are flicking, I don't see it being the ECU. I would start by having the headlight switch checked.

OK,
I am seeing Flashing headlights, is it Flashing like in indicator, or Flickering. Here we can look at many things. I (with due respect), do not see ECU in this, except to re-set ECU and using a Mitsubishi Certified Dealer Scanner as the perfect tool. Apart from all of that. I can see a dying Alternator, which in turn could have the Battery not holding its proper voltage, and vice verse, slack Alternator belt and really simple stuff that can be done by the average mechanic. So look in the direction of your Battery-Alternator-Belt-Headlight sensor-Headlight Relay and or Fuse can be slack and there is a simple to find short. Bottom-line, none of the said parts have yet failed, as the light is turning-on! It seem like a very simple tune-up, and checking of those basics can be your answer. Wish I was there, I would fix it for fun!!!
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Hi Christina , do you daytime running lights ? There is a front ECU that controls the headlamps ! there is nothing your going to be able to do ! you need to take it to someone who has the tools to check this . Could possibly need new ECU - electronic control module !