Headlights dimming/power loss?, Headlights dimming after brakes applied or blinker... Options

[Betta-J]

Honestly, I am stuck on this issue for quite a while.
I initially thought it was the batter or the alternator.
I got the battery tested at Walmart as that is where I bought the battery.
They told me the battery was low but the cells were ok.
I figured it was the alternator. I changed that out.

The other things I changed out were the sparkplugs, although I could not get one of them out and I think it's misfiring.
I'll have to take the head cover out and unscrew the pipes, as the sparkplug that I couldn't get out, will have to be taken out this way due to the nut part being gone(?). I can't really see.
I've changed the water pump and the timing belt at this time too.

The symptoms:
1. I apply the brakes and the lights go dim.
2. I don't have a hard time starting the car, but sometimes I feel that during idle it may go off...it feels weak.
3. The alternator I bought was tested and was ok.
4. I hear my misfiring slightly--but I know which one it is; I just don't see this as the problem, but who knows.
5. I have my hazards on or my blinkers, and it dims as well.
6. When I lock or unlock my car (security keyless), sometimes the horn sounds weak like there's no battery. Sometimes it's strong.

I would appreciate any insight or help on this.
If you need more information to diagnose, just ask and I will try to observe more symptoms.


Thanks in advance.

Betta-J