I found this HDE Bluetooth Receiver on Amazon fairly cheaply. Paid less than $15 after shipping and it's interface was simple with just an on/off switch and a 35mm jack output and a mini usb for charging. Plus the size of it seemed perfect to gut and insert inside the giant headphones
HDE Link on Amazon
(If the adapter moves or is taken off of amazon, google it).
The pager-like case opened pretty easily by wedging a screw driver in the crack to separate the top and bottom.
The original idea I had was to wire the speakers and external jack straight to the board, but the circuit there was pretty small so I gutted a male headphone jack off a pair of earbuds and wired that up accordingly.