I think JD was thinking of the earlier ambient temp guages. For the LCD gauges in the '95 900, the bulb is on top of the ambient temp gauge. The bulb has a blue base with a square end and inserts with a quarter turn into the top of the unit. I don't think you can find it anywhere except Volvo. Sorry, I can't find my receipt with the correct part number. The bulb can be a PITA to get at and replace as there is only a small space above it. The best way is to remove the instrument cluster so you can see what you're doing.

As for the switch bulbs, these have an integral black plastic bayonnet base. The switch cluster bulbs are p/n 1363149 and you won't want to be buying a box full as these are quite expensive (like around $10 each) and of course only available from Volvo. I believe there was a RWD post recently on re-building these bases using LEDs and there is also the option of getting the older style bayonnet bulb bases with the replaceable bulb out of a 740 and splicing the wires. Some bulbs are in the switches and some are inserted in a light tube at the top of the switch clusters. If you try to yank the switch clusters out you can damage the dash unless you press in a spring clip on the outer back side of the cluster and swing the switch cluster out. If you remove the instrument cluster to do the ambient temp bulb then they should be relatively easy to locate and replace without removing the switch clusters.

Read the FAQ and search the RWD forum for previous posts on these topics such as this and this thread.
--
Dave -not to be confused with a real expert