I attach below the manual for this item, kindly note that the bulb for this is only 12 volts not 220/240V without putting a meter on the fittings it is quite difficult to establish which of the two areas it could be in. I would think that all the bult holders are connected together and likely either a wire has broken or alternatively as you say there is a problem with the bulb holders ~ taking in mind that the bulbs are quite high in wattage and will generate a lot of heat I would go for the bulb holders first. Hope that helps ~ Ian
It looks like the item is only available as a four lamp cluster as a spare part with a horrendous price !!