The warning light comes on if the expected load is not met, when switching on the bulbs. If a replacement bulb has not fixed your problem (dim dipped beam, if I understand correctly) then the fault must lie somewhere else. The most common fault would be a poor earth, within the headlamp itself. This commonly occurs due to corrosion of the earth terminals, which in turn can occur when the moisture drains get blocked, and moisture builds up within the headlamps. Do you ever see clouding of the headlamp lenses?