I think you're missing the big picture abicash, to do what you want you'll need to connect to FOUR devices.
The mobile phone, the audio device the intermediary device and the target device.
The intermediary devices would pair with both the mobile phone and the audio source process the information coming from both of them and then send that information to the target device (the car)
So the intermediary device will need to be capable of 3 simultaneous bluetooth pairing connections independently, and you'll have to write all the code yourself for it as I'm not aware of anything like this on the market. You MIGHT be able to do it with a VERY complicated bluetooth dongle setup and an audio mixer outputting to another bluetooth dongle but it would be a lot of effort and might have trouble in an automotive environment.
Basically what you would do is buy two bluetooth audio receiver dongles (tons of them out there) that support the profiles that your mobile phone and GPS do (probably headset device) those dongles would output straight audio, that goes into the mixer which spits the two out to the third dongle which would be a bluetooth dongle pair with the car. The whole setup would live under a seat. It's doable I think but it would probably cost 50-100 dollars and I'm not sure what gotcha's there could be during an install, all devices would have to auto-pair on power up