[papajam]

Although doubling the wattage of two instrument light bulbs from 2W to 4W would double the current draw of these lights (from 0.34A to 0.67A), the total draw would still be less than 4A on the entire circuit, well within the 8A fuse rating (assuming that your wiring is as per the diagram). Additionally, the instrument lights (and some other stuff) are protected by fuse #5 while the gauges and warning lights are on fuse #6. Theoretically, one circuit has nothing to do with the other.
As has been suggested, I'd check for a blown #6 fuse.