The main components are

1) Open Source Single Board Computer (SBC)

SBC to interface with your car's Onboard diagnostics port(OBD-II)

I intend to use one of the open source boards such Raspberry pie, Parallel a etc. I will pick up my choice among the boards listed below in after careful analysis of cost, ease of SW use and availability of add on modules.

The Biggest-Little Revolution: 10 Single-Board Computers for Under $100

http://www.eetimes.com/document.asp?doc_id=1319262

The board will have following

CPU + RAM + Flash Memory

Positioning Peripheral

GPS receiver

Communication Peripherals

GSM/GPRS (quad band)

Bluetooth

RS232

USB Host

2) Open Source OBD Bluetooth Adapter

This device acts a bridge between Bluetooth enabled SBC and your car's Onboard diagnostics port(OBD-II). Technically it interfaces with your CAN bus via OBD port and SBC Host CPU via Bluetooth.

Each car vendor is free to define the data goes over CAN bus in the way they want. In other words, there is lack of standardization among car manufacturers about data that flows over CAN Bus. So the car manufacturers need to make the data from their cars available in a standard format and that the data needs to be easily shared. Towards that end, Ford has started an initiative to standardize data format and sharing of it. They have already defined specs for same and also made a open source OBD Bluetooth adapter. This is a good start towards standardization and instead of reinventing the wheel, I will use Ford initiated the open source data format and open source OBD Bluetooth Adapter.

The Ford Reference VI OBD Bluetooth Adapter or CrossChasm C5 OBD Bluetooth Adapter