And how would i do this? Thanks

Yep sounds like a bad earth, typical symptoms. If the lights worked ok before then certainly, if they have never worked properly it could be because as you say, aftermarket lights not fitted properly perhaps.

Check the earth mate, make sure it's a good clean surface

For some reason when i use the brake pedal when my lights are off (daytime) my side lights come on? Im also have trouble with my reverse lights as they dont seem to be working even after changing the bulbs? Could it be, because i have silly aftermarket lexus lights? Thanks