Have an old Bluetooth headset sitting around collecting dust? Instructables user treyes4 shows off how to convert that headset into a receiver with just a few parts.
All you'll need is a 3.5mm jack, some wires, a soldering iron, lead and a bit of hot glue. You'll essentially pull the old headset apart, solder in the new jack, and be on your way. You'll need a special app to get the Bluetooth working on your phone, but it's a simple (and cheap) way to add Bluetooth pretty much anywhere.
DIY bluetooth receiver for any amplifiers under 3$ [Instructables]