So I switched the ‘01 taillights to my ‘03 gle I swapped the housings no issues as well as the harness piece for the bulbs but now the running lights won’t brighten when I press the break I tried to switch back to the ‘03 harness piece and same results. Help?

 

I did the same with my 02, from what I remember it fit the same as stock. what do you mean by " running lights " ?

 

Are you sure you’ve installed dual filament bulbs in the tail light socket? That’s the only way you’ll have different brightness levels between running lights and brake lights.