Where is the bluetooth antenna located in iPhones?
There are teardown/repair guides at iFixIt, they note where the chips and antennas are located.

For example, in an iPhone 4S teardown, here's a photo of the Wi-Fi/Bluetooth chip.
There is a guide on replacing the upper antenna - which is the Wi-Fi/Bluetooth one.
And there is also a step-by-step guide on fixing the wi-fi which provides the detailed photos and description.

It appears that the Bluetooth antenna is located at the top of the 4S. The discussion of the iPhone 6 teardown also states that the Apple fit the antenna on the top. 

So, iFixit has a lot of manuals and photos to provide. Also, they've got a great community, so if you have any further questions, you can ask them instead.