First, thank you Raja for the reputation :)
Sorry for the delay in replying.
I would like to know whether you have an USB port on your HeadUnit ?
If yes, can you charge your phone by connecting to it?
I carry my Digital Camera (both pocket and DSLR) when we go for a trip. I feel kind of embarrassed while taking photos thru phone. (It's purely personal choice).
As per my limited knowledge, both MW600 and Lumia 620 got the same kind of charging point (Micro USB).
I do not have a USB port on my audio system (Dzire VDI). I have bought a device from ebay (belkin usb charger)
I connect my usb cable of my phone to this and charge my phone on the go.
(There is two model in ebay . One fake and one original. Original got the mat finish with powdered touch. )
If you want to use the bluetooth only in car, you can check the price for that accessory. If it is worth the price and do not protrude outside, i will suggest to go for that. It may look neat than a cable connected to aux port.
If that is above 3K and you want to use bluetooth for your day to day use other than car driving, i suggest something like a stereo bluetooth device.
You can buy a car holder for your Lumia 620 and use it for navigation. You get turn by turn navigation on speaker with music playing together. Music volume gets lowered automatically when you get alert from navigation software (In android, it says so. Hope the same in windows 8 too).
Do not buy cheap car mounts/holders as it may not work well (it may damage your phone by falling from windshield).
My final suggestion : Do not jump in to buying anything as someone suggested (eg:me). Take your time. Go to some store. See it. Try the navigation and music same time playing on speaker /headphone. Do not buy anything unless you are fully convinced. Please. It is your money. (Hard earned?)