I noticed that the left side of my speedo wasn’t as bright as the right side, so I did a bit of research and asked a few friends (all 993 owners of course ;-) on how best to go about changing an instrument bulb. 
In short, it’s best to check what bulb is blown first. The fitting type is B8.4D, and it’s likely to be either 12V 0.9W or 12V 1.5W. Both bulbs are available from Porsche (96464192000 and 99963102790, Fits: Porsche 964 1989-94 and Porsche 993 1995-98).
Lots of people have upgraded to LED bulbs (given their brighter and whiter appearance) but I wanted to keep the gauges stock as I actually quite like the retro look! Superbrightled.com comes highly recommended though if you're into the modern look.
First things first - removing the speedo. They can be pressed in quite tightly so you’ll need to carefully work them out - I bought myself a Car Trim Removal Tool Kit so as not to damage the dash or the gauge. Simply get behind the rubber seal and apply some pressure to ease it out. Take your time, and like me you'll probably work your way through a few tools before you find the right one. Once loose, you can simply unclip the wiring block at the back of the speedo to fully release it.
The culprit was a 1.5W bulb (ALBA) - in fact all 3 bulbs were exactly the same. So instead of ordering from Porsche (minimum order of 5 is £50.88 inc VAT!!!) I spotted these Osram equivalents for a fraction of the price. Bargain! I took the opportunity to change all 3.
One tip - I’d advise against using a small flat head screwdriver to turn the bulb, as the plastic base isn’t all that hard so the head of the screwdriver will cut into it. Instead use a 6mm socket on a straight handle, push down and then turn - it makes life so much easier.
Another little job done! And I’ve got enough spare bulbs to last me a lifetime ;-)
