Today I succeeded in adding always-on 12V sockets to my '05 Prius, thanks in part to specific advice from Ray O and Hachiroku and general guidance from others I appreciate you all.
The Prius comes with two 12V power sockets. They are conditioned on the ignition. If the car is powered down, so are the sockets. That's fine for a radar detector, say, but cellphone chargers or solid-state coolers and such should be powered continuously.
Hachiroku pointed me to Littlfuse's Add-A-Circuit fuse tap. I'd never seen one. It's a brilliant idea cleverly implemented. It made the job easy. Hats off to Littlfuse. The kit includes the tap and a selection of mini-fuses for protecting the device you're adding. At AutoZone it sells for under US$7.
It turned out I didn't need to tap into an existing circuit. In the fuseblock located above the driver's knee I found a continuous 12V at positions 27 and 28. There might be others; I didn't look further. If your options list differs from mine, 27 and 28 might already be used, but with Add-A-Circuit that's no impediment.
For ease of access I removed the plastic panel at the sill of the driver's door and the cowling just forward of it. They just pull away with a gentle tug. Under the cowling is a hex-head bolt that makes a great ground for the unfused side of the new circuit. With the cowling off you'll also get a good view of the parking brake ratchet -- about the only physical threat to new wiring in that area.
The one downside to the Add-A-Circuit approach is that you won't be able to put the fusebox lid back on. I concluded it's no big deal, but I immediately put the lid in the glove compartment. Its attached diagram is the owner's only clue to the functions of the individual fuses.
This project has freed up the car's built-in sockets and has greatly neatened the interior's appearance. The job was a satisifying morning's occupation.
Brent