Not sure what you bought on amazon, but most of them I've seen are host devices, not clients. hosts (like speakers, headsets and your car) can't talk to each other, only to clients (like phones).
Jabra used to make a device (a210) that will do what you want, but it is hard to come by these days:
I used one with my Kenwood TH-6 in VOX mode, it worked well, but I accidentally connected the wrong charger and destroyed it.http://radcomms.net/Jabra_A210.html
has some good information about it.