Redsn0w 0.9.14b2 downgrade from 06.15.00 to 05.13.04 baseband on iPhone 3G/3Gs
Redsn0w 0.9.14b2 features a baseband downgrade option for iPhone 3G/3Gs users who are on the 06.15.00 iPad baseband. This baseband notoriously causes loss of GPS functionality, so users will now be able to downgrade to 05.13.04, another Ultrasn0w compatible baseband, so that they can unlock and have GPS.
Step 1: Download Redsn0w and the current firmware version you are running from the links above.
Step 2: Open Redsn0w and go to Extras>Select IPSW and choose the firmware file you downloaded in step 1. 3Gs users with the new bootrom can allow Redsn0w to fetch the firmware files but it is recommended that they perform the first 2 steps as well.
Step 3: Turn your device completely off. Hold the power button until the "slide to power off" switch comes up and shut it down. It is very important that your device is turned off when you initiate the process to avoid issues.
Step 4: Go back to the main screen of Redsn0w and choose the Jailbreak option. When you get to the options screen check the "Downgrade from iPad baseband" option. If you are jailbroken already, be sure to uncheck the "Install Cydia" option. If you are not already jailbroken, you can leave that box checked. Click next and enter into DFU mode per the on-screen instructions.
Step 5: Redsn0w will take over from here, you will see the normal Pwnapple screen, please note that once you get to the "Flashing Baseband" step, it will take a very long time to complete(3-8 minutes). You also will not receive any on-screen feedback during this process, do not interrupt the process just let it finish.
Step 6: Once the process has completed you will now be on the 05.13.04 baseband and you will be able to unlock with Ultrasn0w and have GPS capabilities.
For those on an uncompatible baseband other than 06.15.00
If you are on a different iPhone 3G/3Gs baseband that is not compatible with Ultrasn0w, you will first have to install the iPad baseband with Redsn0w and then proceed to downgrade to 05.13.04. Please note that iPhone 3Gs users whose devices were manufactured after the 34th week of 2011 MUST NOT install the iPad Baseband. Doing so will brick your cellular radio and you will not be able to use it as a phone any longer. You can tell when your phone was manufactured by looking at the serial. The 3rd, 4th and 5th digits are the date. The 3rd digit is the year and the 4th and 5th are the week. "135" would be the 35th week of 2011, so anything on or after that serial cannot use the 06.15.00 baseband.