brake light on one side dimmer

Hi
my n/side brake light (facelift model 2004 TD4) is dimmer than the other side when they both come on, changed the bulb on that side and both complete units have been changed recently, I think its the bulb holder itself but just seeking assurance...btw anyone know the part number ?
cheers

have a look inside the holder, sometimes the contacts scorch and it's pretty obvious...

Or is the bright one faulty, is picking up the brake light element? Check the earths, especially if a towbar is fitted, could also be crud in tow electrics(if fitted)

no tow bar fitted, just dimmer on the n/s, cleaned the contacts in the holder, side lights look pretty similar but brake light on looks quite a bit dimmer than the near side bulb when pedal pressed, something stupid but cant quite put my finger one it.

Get a multimeter on the bulb connectors both sides and compare them to see if there's a voltage difference...Could be a bad earth causing a higher resistance to the dim one