What are these later cars (mine happens to be a UK 94 Spi so no AC) alt supposed to be putting out? With my wipers, blower motor, and lights on, it seems mine can't really keep up. With the lights and interior fan on, when the wipers go (I installed intermittent wipers) I can hear the fan motor dip.
With everything switched on I'm not surprised that happens, particularly at idle speed.
Check or change your belt incase it's slipping, other than that just carry on.