I was able to replace the driver's side side-marker bulb, but I am finding it impossible to get a grip on othe socket assembly on the passenger side. I have moved the air intake hose out of the way and I can (barely) reach my hand in and grasp the socket, but it will not budge. Any ideas?
Welcome ! It is a definite challenge. There was a video on YouTube of a guy taking the front bumper off to get access to the housing, then pulling the housing off. I was able to change it in place, but it ain't easy. It does require a fair bit of contortion and patience if attempting to do in place. A challenge, but possible. Good luck.
The 2010 manual (page 478, see attached image) says to remove the bolt and unclip the engine coolant reservoir hose. Then lift up the air cleaner inlet. To turn the base, try needle-nose pliers for leverage. They make it sound easy, don't they? When they say have the dealer change any other lights not mentioned in the manual, imagine how difficult those are.
I recently replaced all the interior (domes front & rear, vanity, puddle-lights, cargo area) tungsten-filament lamps (both regular wedge and festoons) and all the markers and both number-plate lights (grand total of 15 bulbs) of my 2009 Gen II with LEDs - total time was less than an hour. I bought an LED kit from eBay ($5.99 inc. shipping). Here you are: 15pcs White SMD Map Dome Door LED Lights Interior Package Kit for Toyota Prius | eBay
Thanks for the heads-up! I still have all the original bulbs, apart from one of the number-plate lamps which had failed, but I'll keep an eye on them!
In case it helps someone in the future, I found that in order to get an OK grip on the right front marker (parking) light's socket, not only does the coolant tank need to be moved away, but the main headlamp socket needs to be moved out of the way. Getting that main bulb, its socket, and its wiring harnes out of the way mad just enough room for my right hand to push and twist the marker socket back in place. I did it with the parking lights on, so I could verify the light shown through the middle window of the orange side reflector lens. It took multiple tries, since I did not know the proper orientation of the big tab. So I twisted and pushed until a clockwise turn left it snug.