Rep Power: 0

I have a 2000 Honda civic EX that I just purchased a month ago, its seems that the dashlight, headlights, brake lights and interior lights pulsate/flickers randomly for 3-10 seconds at idle.

I took it to the dealership and they told me it was the battery, got a new honda battery, same thing...

they told me that it was now the ELD and the whole fuse box needs to be replaced with this year, got it and still the same issue.

They already checked the alternator and it was fine, I added more ground wires to the engine bay "the big 3". Lights are fine while driving, only when i'm in idle the issue starts. Stock everything including radio.

 
Rep Power: 518

I think 3-10 seconds is more frequent than the ELD system would normally operate, so I doubt it's that.

Have you used a voltmeter to measure system voltage as lights get bright and dim? What are the high and low ranges of voltage during this?

Does it have a replacement alternator on it? Aftermarket reman alternators with cheap internal electronics can sometimes do this.