Bluetooth again

Hi!

Does anyone know of a car bluetooth adapter that would switch/override the car stereo at line level as opposed to taking over the speaker connection? All the ones that take over the speaker are of insufficient power to be heard on belgian motorways in an open car with 1930's aerodynamics...

That seems to be a really tough combo to find. Did you have any success?

The best idea I had would be to take one apart that had an LED which lit during a call and use that to trigger a relay to switch the RCAs over.