How do you change the rear window brake light bulb on a 93 Honda

If you have already checked fuses and bulbs, you can do the following:
If you have a multimeter, set it to DC voltage 20 and remove the light bulb. Have someone else press the brake pedal and touch your leads from the multimeter to the contacts (one lead on one, the other on the opposite terminal) and see if you're getting 12-14 volts DC. If you are getting the proper voltage, then the bulb should be replaced with a known working bulb.
If you are getting no power when the brake pedal is depressed, you have a break in the wire somewhere and will need to track it down.
Sometimes, there is a separate fuse for the 3rd brake light - have you checked for that fuse?
It's possible the wire leading to the 3rd brake light has been cut or otherwise damaged - possibly from overloading the trunk and items physically damaging the wire.
Another possibility is a faulty ground for that light assembly. Try to locate a service manual for the car and track down where the ground wire should be and make sure you have a solid connection.

If all else fails, take your car to a qualified technician for a closer look.

The third or High Mounted Brake Light runs straight off the brake light switch on the brake pedal arm. You say you tested the "brake unit"? I assume you mean the third brake light assembly, and if you get power there when someone steps on the brake pedal, then the bulbs are bad.

The power flow for the brake lights goes like this: Fuse ->Brake switch -> Turn signal switch -> Rear lights. The only difference is, if you have a 3rd brake light in your rear window, that gets its power directly from the brake switch. It does not go thru the signal switch first. So, if you are having a brake light problem, and your fuses are good, check the 3rd light in the rear window, if it lights up (assuming the bulbs are good, no one ever changes them), then the problem is most likely in the turn signal switch, hope this helps!

Look under the dash, on the brake pedal arm. There are one or two switches that activate brake lights and/or shift unlock. Check for loose mounting nut or a missing rubber nubber. Is the brake pedal coming all the way back up? Look on the floor for something not usually found there. These switches are adjustable and it may be as simple as adjusting them. Have a helper look for brake lights on and shift lever unlocked while you push and release the switches with your finger. Depress the brake pedal to get it out of the way. Remember to set the park brake first. Could just be a bad switch or loose connector. Check fuses and the bulbs. Is shifter fully in Park? Also try jiggling the turn signal lever.

The power flow for the brake lights goes like this: Fuse ->Brake switch -> Turn signal switch -> Rear lights. The only difference is, if you have a 3rd brake light in your rear window, that gets its power directly from the brake switch. It does not go thru the signal switch first. So, if you are having a brake light problem, and your fuses are good, check the 3rd light in the rear window, if it lights up (assuming the bulbs are good, no one ever changes them), then the problem is most likely in the turn signal switch, hope this helps!

Yes they are. The power flow for the brakes goes like this: Fuse ->Brake switch -> Turn signal switch -> Rear lights. The only difference is, if you have a 3rd brake light in your rear window, that gets its power directly from the brake switch. It does not go thru the signal switch first. So, if you are having a brake light problem, and your fuses are good, check the 3rd light in the rear window, if it lights up (assuming the bulbs are good, noone ever changes them), then the problem is most likely in the switch, hope this helps!

This is a simple bulb replacement.Simply pop off the access cover(if you need to) then twist the bulb in a counter-clockwise motion to remove it.Install is opposite of removal.I hope I could help, rate me accordingly, Thanks.

check the fuses
than the relay's its one of these

The right brake light sensor is located in the trunk area by the right rear taillight assembly.

check the fuse for the stoplights. Even though the 3rd one is working it should be on a different fuse. With both of them not working even after you changed the bulb it would seem to lead to a fuse.

Problem could be the Brake Light Control Module. That's about the only other thing other than a bad wire somewhere if the bulbs and fuses are good. 91 Accord has 2. One on each side in the smaller of the 2 rear tail light lenses. In order to change the module you have to remove the smaller of the rear tail lights. Drivers side is 5 pin. Passenger side is 6 pin.

do you have a 3rd brake light like on that is on the roof or in the rear window? if so does it work?
the brake lights are usually a 2way bulb so just because the lights work doesn't mean that brake lights will. if the 3rd brake light works replace the brake light bulbs if none of them work including your 3 brake light. check into having your brake switch changed. that is the switch under the brake pedal. when it is pressed the brake lights come on and when released they go off. if the bulbs do not fix the problem than try the switch.