Maybe a stupid question here but I can't seem to find a way to disable the internal amps.

I'm using the F900BT with an external 5ch amp.

I left all the speaker wires unconnected but it didn't feel right? Or?

Is there an auto-detect feature?

You can't, and don't worry about it.

Just don't hook up any of the speaker wires and remember to cap off the speaker wires so there's no chance of them touching any metal or each other behind the dash. That's all there is to it.