In reply to this and the PM, You don't have to go deeper than removing the instrument cluster. No further disassembly needed. The bulbs in a 1998 should be easy to remove and replace with a small screw driver. I'd recommend replacing the three that are marked with the red line in fig. 15 while you have it out. Removing the cluster is not difficult, but the plastic connectors for the wire harness may be brittle. Handle gently.
Here's the location of the odometer LCD illumination bulb:
Here's a pic of the same bulbs on Amazon, but listed for a BMW. I couldn't find them browsing PelicanParts, but customer service may find them and confirm fitment if you call. "12 volt 1.5 watt". Part number in the Porsche catalog is: 999.631.303.90
