Just in case anyone else forgot their password for their X850BT, here's how I got a hold of mine.
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First, some disclaimers:
If you stole your unit, please give it back, save money, and buy your own!
Otherwise, this is my: RUN TESTMODE AT YOUR OWN RISK statement!
I searched this forum and Google'd like crazy, and there, to my knowledge, is no other documented method for retrieving the AVIC-X850BT's password specifically, sorry if this is a repost.
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The way I figured it out is as follows:
[FYI: I POSTED THE LINKS TO EACH THREAD I RESEARCHED BUT PROVIDED THE LINKS IN THIS HOW-TO SO YOU DON'T HAVE TO BROWSE AWAY FROM THIS PAGE! 
]
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STEP 1: Get the right Testmode file.
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I went here:
http://avic411.com/index.php?/topic/36681-avic-x50-hack-general-topic/?p=266663
and downloaded the link (testmode-x50v2.zip) pionara posted here:
nag screen removal will come later, first we must get testmode working !
ok this is attempt 3:
testmode-x50v2.zip
also, wondering if the hidden, in-app debug/service menus work on x50 series. Can someone try it out?
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STEP 2: Get a functional testmode base.
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1. I visited this thread:
2. and downloaded the following link:
Condi's HACKMODE v1.4 BASE - INSTALL VIA TESTMODE.zip 5.32MB 6140 downloads
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STEP 3: Extract the files onto the proper medium IN THE RIGHT ORDER.
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1. I took the "Condi's HACKMODE v1.4 BASE - INSTALL VIA TESTMODE.zip" and extracted ALL of it's contents to a BLANK miniSD card.
2. I extracted the contents of testmode-x50v2.zip onto the miniSD card and OVERWROTE the existing testmode.key file.
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STEP 4: Startup in TESTMODE and access the right file.
PATH:NAND\SYSTEM\SECURITY\SECURITY.DAT
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1. I then removed the miniSD card from the computer and placed it into the AVIC-X850BT.
2. I turned the vehicle in to ACC Mode/Started vehicle.
3. The unit then successfully booted into testmode.
4. I used the volume keys to navigate the menu and selected the File Maintenance option.
5. I used the volume keys to navigate to the FIRST NAND folder (it's after the HDD folders).
6. I then navigated to the SYSTEM folder.
7. In the folder is a SECURITY folder, select it.
8. There should be several files in the SECURITY folder, among which is the SECURITY.DAT file, select it.
9. Navigate to and press the appropriate key to "[TXT DUMP]" the file.
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STEP 5: Accessing your forgotten password.
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1. Following the "[TXT DUMP]" of the SECURITY.DAT file, the top left corner of the screen will supply you with 3 things: YOUR forgotten password, your password HINT, and a buncha shenanigans.
2. I recommend taking and picture of that screen with your smartphone or other mobile device and then turning off your vehicle.
3. REMOVE THE miniSD card!
4. Turn your vehicle to ACC or turn it on.
5. After your AVIC-X850BT boots, try to use the picture to interpret and input your password.
Hope this helps someone! I drive around for two weeks, getting mad every time I so much as LOOKED at my radio lol.
GOOD LUCK!
