2005 Prius - left tail light: It appears to be out but. However, when you get really close to the housing all the LEDs light up but are very faint. Dealer wants to replace the entire housing. I dis agree as is the LEDs are still lighting, to me these are still working, right? If true how do I fix this?

The bulb isnt bad to replace. Is it HID? There is a round black plastic cover that needs to be unscrewed. A dealer normally charges .3 to change a bulb. If youre having difficulty, pay a professional. Its really not that difficult.

You have a problem in the wiring at these bulbs. Since all of them aren't working then the problem is between the bulbs and the ground: the wires that go from the bulbs to the body of the car are either no longer attached to it or the connection(the bolt that tightens them on the body) is covered in rust and that insulates it.
Find the connection bolt by following the wires, clean it with wire wool then unscrew it an clean the copper connectors from the wires and the body with fine sandpaper until the metal shines. put all back together and cover it in grease to protect it against future damage.
BTW you can't get the bulb out because the corrosion has got in there also: just force the bulb out and then clean all the bulb slots with sandpaper until they shine. Don't forget the brass blades from the bottom of the slots, the ones that make contact with the plots on the bulbs.

While leds do not burn out like fluorescent lamps and other bulbs they will however degrade and dim over time.
no bulbs to replace just the assembly.
tis not hard to do... youtube how to replace 2005 prius tail light.
a officer can pull you over if it is not emitting the proper lumens.

There should be at least 2 bulbs in the light assembly. One light is the light that is on when you turn on your headlights with out the brake pressed, the other light only lights up while you press on the brake. Make sure you are replacing the correct bulb. You may have been replacing the bulb that stays on. Your lights may be set up to turn off the light that stays on when the headlights are on while pressing the brakes.

A new unit is $56.00 on EBAY. Replace it yourself
Google 2005 Prius left tail lamp

Replace the right rear brake lamp. The bulb possible use two filaments one for brake and tail light. One burnt out filament in one bulb.

Th other post is incorrect, the 2004 has LED brake lights that are part of the tail light assembly. you have to replace the whole assy. There are two screws that hold the assy in, they are both covered by white cable clips. The clips just pull off and then you can use a 10mm wrench to remove the nuts on the screws. My brake light stopped working and during the removal, the LEDs started working again, hopefully you will find the same situation as the LEDs are supposed to last for years.