Sorry, i've been busy, but I've had a number of requests for this, so here ya go -- from memory.
Getting into the Service mode on AVIC-D1 (and D2?)
1. Turn car on, AVIC unit should power on.
2. Hold down OPEN button so that the panel flips down all of the way (as if you were going to replace the Map DVD
3. Using a paperclip (etc), press the Reset button on the LCD panel -- this is inside a small hole next to the directional (joystick) control. Hold down the button.
4. While holding down the reset button, press the DVD drive Eject button (the lower of the two eject buttons, it says "ROM" on it). Hold it down.
5. While holding down the Eject button, release the Reset button.
6. The screen should now say "Please press the [RESET] button." It's lying. Don't do that. You can release the eject button now.
7. Instead of pressing the reset button, enter this code using the joystick:
Up, Up, Down, Down, OK (where OK means "push the joystick in")
8. You should receive a message that says "Password Accepted", and then be in the service menu. If you'd like, you can press the Open button to fold the LCD panel back in.
Reflashing firmware, to change the message text, etc. This is from memory, let me know if you can't figure out what to do from here.
First, prepare a CDR(W) or DVDR(W) with the updated firmware. The easiest way to do this, I found, is:
1. Insert a Map DVD into your computer.
2. Use any program to copy all of the files into a directory on your hard drive.
3. Go into that program and delete all of the large files, and probably all of the directories. In theory, you only need a few of the files -- the ones that contain the firmware -- but I never bothered to figure out which. Instead, just delete enough of the large map-date files such that the resulting files will fit onto your 650MB CDR or 4.7GB single-layer DVDR.
4. Edit the firmware files in that directory, as appropriate
5. Burn the contents of that directory to a CDR or DVDR using any program.
Flashing the firmware:
1. From the main service menu, move the joystick right to get to the second page of the menu
2. Choose "6. Program Forced Write"
3. Choose "3. Application Program"
4. Insert your modified CDR or DVDR into the appropriate drive; if necessary, use the OPEN button to flip the LCD panel and/or use one of the eject buttons to eject a disk already in the drive.
5. Next to option "2. DVD/CD-ROM", it should give a version number and not "NG" (No Good). If so, select it and press OK.
6. Select the appropriate language, in my case "2. English US" with the joystick.
7. The help text at the bottom of the screen directs you to make your selection and press the "[NAVI]" button -- on our units, this is the MAP button in the upper-left corner of the display.
It will then take about 90 seconds to read the firmware off the disk and flash it. When it's done, it will say "100%" for both stages, and you can hit the eject button and remove your modified disk. Then, hit the reset button and watch the unit boot with your new firmware.
If you have modified any of the firmware files without recalculating the checksum as I described earlier in this thread, instead of saying "100%" it will say "NG", IIRC. If you then reset the unit, it will perform the checksum verification upon boot and then display a message saying something like "An Update Is Required, Please Insert Map Disk". If one is already in the drive, it will automatically try to reflash itself. This means that you shouldn't be able to brick the unit by just reflashing the "Application Program".
Now, all that is done by the "System Program", so I'd avoid touching that, if I were you 
Hope this helps. -b
