You have seen my setup from the "external view". My radio is inside a metal box, so I had to add the external BT antenna. The antenna itself is just a generic 2.4GHz stubbie with male SMA connector.
I added an external SMA connector to the rig, and ran a mini coax to the point where the internal antenna connects to the BT module.
The internal antenna is just a meandering copper trace etched directly to the front panel circuit board. The circuit board itself doubles as the front panel, there is no separate metal or plastic front face, just a plastic sticker covering the PCB.
(ballpoint pen for a size reference)
Yes, adding the external antenna connection requires cutting and soldering into the front panel PCB.
One more picture of the mess before if went into the enclosure.