Hello and welcome.
By your post, i am thinking that you mean that when the sidelights are on, the passenger side 'dip' bulb is also glowing slightly and when you turn on the dip, everything works as it should but for clarification purposes, we need to ascertain exactly what you mean by 'dim' headlights and 'brights' and secondly, what lamp in question is not operating properly.
Your dashboard switch has 2 positions (ignoring the foglamp positions) 1st position is sidelights - these illuminate the 'inner' lamps (nearest the grille) with a small bulb giving off a fairly dim glow. The 2nd position is dipped headlamps and illuminate the 'outer' lamps (nearest the wings) with a much brighter light. Main beam is activated by pulling on the left hand 'indicator' stalk and illuminates the 'inner' laps with a high power bulb, giving off a very bright light.
If my first sentence is correct, that the dip bulb is also glowing slightly with the switch position on sidelights, my first thought would be an Earth fault, possibly caused by a faulty bulb - probably the sidelight bulb and i would start by removing both sidelight bulbs and turning on the sidelights to see if the problem persists. If the problem is cured with no sidelight bulbs in, you could put the sidelight bulbs back in in the opposite lamp sockets (left to right etc - and no need to fit them back into the lamp holders at this stage) and if the opposite dip lamp glows dimly, this would confirm the issue and just replace both sidelight bulbs with new ones. If that does not work, try swapping over the dip headlamp bulbs (left to right etc) and again, if the opposite lamp now glows dimly, the dip bulb will be the culprit and if nothing changes, its probably a fault in the wiring.
I do suspect a dodgy passenger side sidelight bulb though.
By your post, i am thinking that you mean that when the sidelights are on, the passenger side 'dip' bulb is also glowing slightly and when you turn on the dip, everything works as it should but for clarification purposes, we need to ascertain exactly what you mean by 'dim' headlights and 'brights' and secondly, what lamp in question is not operating properly.
Your dashboard switch has 2 positions (ignoring the foglamp positions) 1st position is sidelights - these illuminate the 'inner' lamps (nearest the grille) with a small bulb giving off a fairly dim glow. The 2nd position is dipped headlamps and illuminate the 'outer' lamps (nearest the wings) with a much brighter light. Main beam is activated by pulling on the left hand 'indicator' stalk and illuminates the 'inner' laps with a high power bulb, giving off a very bright light.
If my first sentence is correct, that the dip bulb is also glowing slightly with the switch position on sidelights, my first thought would be an Earth fault, possibly caused by a faulty bulb - probably the sidelight bulb and i would start by removing both sidelight bulbs and turning on the sidelights to see if the problem persists. If the problem is cured with no sidelight bulbs in, you could put the sidelight bulbs back in in the opposite lamp sockets (left to right etc - and no need to fit them back into the lamp holders at this stage) and if the opposite dip lamp glows dimly, this would confirm the issue and just replace both sidelight bulbs with new ones. If that does not work, try swapping over the dip headlamp bulbs (left to right etc) and again, if the opposite lamp now glows dimly, the dip bulb will be the culprit and if nothing changes, its probably a fault in the wiring.
I do suspect a dodgy passenger side sidelight bulb though.
