My daughter's iPhone 6 had bad GPS signal ( would not work with maps), weak and unreliable bluetooth, poor wifi signal. Her phone has been messed up for 2 months. I also replaced the Main Logic Board Antenna Flex Cable 2 days ago and all is restored back to normal. Bluetooth works great, GPS works again with maps, WiFi works great. The Logic Board Antenna Flex Cable is a 1 inch long ribbon cable that is F shaped and a picture of it appears earlier in this thread. I found it online for about $6 which included the shipping and an iPhone tool kit.
https://www.amazon.com/WELLENT%C2%AENEW-Logic-Antenna-Replacement-iPhone/dp/B012ZSSKLA/ref=sr_1_3?ie=UTF8&qid=1461868765&sr=8-3&keywords=NEW+Main+Logic+Board+Antenna+Flex+Cable+Replacement+Part+for+iPhone+6
The logic board antenna flex cable is located on the back side of the main logic board so you have to completely remove the Main Logic Board which is a chore but doable. I used the instructions for removing the Main Logic Board that are also posted earlier in this thread. There are a number of tiny screws of multiple sizes and so some organization of the screws is useful. I used the following hint: Take a blank sheet of printer paper and tape it to the countertop (so it will not slide around and mix up your screws) Draw a 1-2 in square box for each of the steps involving screw removal. In each box draw a circle in the position of each screw location for that step and put each screw in the appropriate location when removed. I also labled each box with the step number and the number of screws for that step. The steps are in the instructions earlier in this thread for removal of the Main Logic Board.
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Once the logic board is removed, turn it over and pop off the 4 connectors on the F shaped Main Logic Board Antenna Flex Cable and snap on the 4 connectors on the new flex cable to install on the board. Follow the steps in reverse order to reassemble. It took me 2.5 hours to complete the job but I also replaced the touch screen/digitizer as well because the glass was cracked. I could have done it in about half the time if I did not replace the front screen. This is an inexpensive fix for this big problem.