I am trying to use a portable BT speaker with my car BT enabled receiver. The receiver is a BT Input device so I think the speaker must be a BT Output device to enable pairing. Advice appreciated.
Speakers can't be an input device because they don't contain a source of audio.
What you can do is connect a bluetooth transmitter to the preamp out of the car stereo. If it doesn't have one then a speaker to line level converter add on.