are all of your dash lights out at night, or just the one behind the speedometer?
Many cars have a dimmer wheel switch, so that you can dim/brighten the interior lights when the headlights are on. Look for this switch, and turn the interior lights back up so that you restore brightness to the speedometer and the other dim dashboard lights.
If it is just the light behind the speedometer and nothing else, then yes you should be able to just replace this single light. If you are friendly enough with your local Toyota parts (not service) department, they might just show you the diagram on how to fix it yourself. It might even be listed in the DIY section in your owner's manual. (If you do not have the owner's manual, log into
http://www.toyotaownersonline.com/ and you can view the section there.)