HID Headlight Back Cover won't open - any tricks?

It is a PITA, the driver's side is worse but change both because the other will start exhibiting the same problem. I got mine (a few times) from EBay, genuine OEM parts and half the cost of what Toyota is going to charge you. Gen II Prii included a 2004 rear ended and totalled in 2012, replaced with a 2009, hit and totalled by someone crossing over the line last Feb. Wife and younger daughter each have a 2006.

It helps to move the windshield washer tank out of the way a bit. There isn't really a tool other than somehow getting both hands on it so you can push down on one fin with one hand and up on an opposite fin with the other. You can try pushing on a fin where it contacts the cap with a big flat head screwdriver. It takes constant pressure, not sharp banging, there is an O ring in the cap. You are SOL on the driver side, nothing to move out of the way, I hope you don't have big hands ;D

When it is time to put it back on, you don't have to get it quite so tight. If you have a small "mirror on a stick" put it down there after you get the cap off to see what things look like. You have to pull the silver connector off the back of the bulb then release the 2 clips to get the bulb out. It REALLY helps the first time to see what they look like before you stick your hand in there. Also note the orientation of the bulb when you pull it out, you can touch the glass on the old one but do NOT touch the new one.

 

I've heard that tapping it with a block of wood can break the O ring loose. What I ended up doing was removing the whole module and working with it off the car.

 

Remove the headlight assembly.
It is much easier to get leverage and makes it less likely to break the fins.

Also makes it much easier to remove old bulb and install the new one.

It takes a bit longer but is well worth it.