1993 Celica GT Hatchback - I need to remove the entire tail light assembly across the back down to the metal. I have already removed the interior trim panels. What is the best way to proceed?

Typically, you need to release interior trim, to gain access to several thumb nuts (can be turned by hand). These nuts retain the entire assembly, and need to be released to access bulbs.

If the 2003 is like my 2002 Sienna and you are trying to replace the light within the hatch, then you have to access this light assembly through the rear interior panel. You can not remove the lens or the light assembly from the exterior of the vehicle. This really did take about 10 minutes.
Step 1 - remove the 2 screws from the hatch back handle by flipping the covers down to access the screws. Insert a screw driver about 1/16 of an inch into the top of each end of the handle to flip the covers down. This will expose the screws.
Step 2 - Use a socket drive wrench to remove the 10 mm screw from each end of the handle.
Step 3 - Use a screw driver to slide the cover up and off of the retaining clip on the left (drivers) side of the rear panel and remove the 10 mm screw from the clip.
Step 4 - Now the only thing holding the back interior panel on is about nine plastic clips. Carefully pull the panel away from the door starting at the bottom where the handle was removed. Work your way across the bottom and up the left side popping each of the snaps loose. Move back to the right and in my case the panel fell off in my hands. I was able to remove and replace the light with ease at this point.

Pretty basic stuff...open the trunk and move the trunk liner material away from the rear of the car to expose wiring for the tail lights...there are 3 light bulbs per side, the backup, brakes and tail lights...just twist the plug where it connects to the lens assembly and the socket will come out...remove the bulb and take it to the auto parts store to make sure you purchase the correct replacement...if your lenses are broken, there are 6 nuts to remove that lets the entire lens assembly be removed and replaced with one from a junk yard, you cannot buy these new anymore...good luck...

are you talking about the brake lights or the light in the back window

Pull back trunk interior trim material. You should be looking at the rear of the taillight assembly. The lights are attached to plugs that twist about 90 degrees CCW to remove. All lightbulb access is from the inside of the trunk. If you need to replace all lights please do so one by one so you don't lose track of which one came out of it's place.

The entire bed line will need to be removed. The long screws that hold the metal bracet where the bed extender attaches also hold the tail light assembly in place.
I pulled the bed liner up at a 45 degree angle and layed it on top of the cab. This revealed the screws. Take both screws out and lay to the side along with the metal bracket for the bed extender.
Now firmly grab the edges of the tail light assembly and pull...there are 2 hard rubber nipples that snap in to hold the assembly. You will feel like you might break it but you won't..just pull straight back hard. The assembly will "POP" out and then you can just twist the back of each bulb there are (3) and pull out..now you will see the bulbs..just pull the bad bulb straight out and push the new one into place. Now just reassemble. Total time will be 30-45 min. for this repair.

The bulb is removed from the back side & the lens is sealed to keep it waterproof and shouldn't be removed.

On most of these, you have to get access from behind, even if it means you have to remove the unit itself, or parts of the car, to gain access to the rear. Once you do, it's an easy replacement.
good luck.

Drain the tank and remove it from the car. Note carefully the position of the hoses and wires going to it. Clean the top of the tank well and dry it. Using a non-metallic tool remove the lock ring holding the tank unit in. Carefully lift out the assembly - there are several items connected. Remove the old pump and filter. Installation is just the reverse.

inside the trunk behind the tail light,pull the carpet down and remove the screws,nuts,bolts or the finger nuts and the light will pull right out

If there is no "access panel" at the rear panel, look to see if there are any exterior screws around the lamp housing. If not, you have to remove the interior trim panel(s) from inside the car. Under there, you will find 3-4 8 or 10 millimeter nuts. Remove them, and pull the whole housing out.