[MTCD Android Head Units General] SkyBeam2048:
OBD is a different interface for car diagnostics. It's not related to CANBUS. OBD is a port and not a bus system as of my knowledge. It allows you mainly to read diagnostics data like error codes etc. Of course the data you can fetch from the systems might overlap so some data might be available via OBD as well as CANBUS.

As such OBD is mainly used to fetch information from the car control units. So applications like Torque can poll for updates regularly. CANBUS works like a bus system where multiple participants send messages to each other via a common data bus. So when you press a button on your steering wheel there will be a "broadcast" message sent on CANBUS about your actions. All devices connected to the CANBUS can receive this information and trigger actions based on it. In your case your CANBUS converter will listen for keypresses and translate them into resistor values on the wires connected to your head unit. However it will just ignore CANBUS messages like "door opened". Such messages might be possible to query via OBD too if the event is stored in some location. But this would require your car to keep track of such events and it would require active polling. This is inefficient and causing delays. Just assume your head unit would have to ask your control units via ODB 20 times a second if the door open event did eventually occur. You don't want this. This is what the message-based CANBUS is made for.

If you manage to get a real CANBUS decoder for your car type then this decoder will forward all the events it understands to your head unit so it can get messages pushed from various CANBUS sources. Your head unit then needs to set up actions to be invoked based on the CANBUS messages fired.

Most MTCx head units have dedicated inputs for events like "parking brake" or "reverse gear". Some cars provide those events on the CANBUS as well as thy provide traditional 12V triggers. So even if you don't have a CANBUS decoder for your MTCD unit you might be able to trigger the reverse camera via 12V trigger input. In worst case wire it up to your reverse light so the MTCD device knows when the reverse gear is activated.

However if you got a matching CANBUS decoder then no 12V trigger is required as your "reverse gear" trigger is fired on CANBUS and your head unit will be notified about this via the CANBUS event firing. So you won't need any 12V trigger line any more.

Some cars provide even more data on CANBUS like climate controls and external displays and allow then for more tight integration. However this is often very car specific. This is why you need a CANBUS decoder matching your car model. As I can see on the picture you shared your car will provide pretty basic information on CANBUS only. Think about if it's worth the hassle and if those information really adds value to your head unit.

Personally I don't have a car using CANBUS but I always wondered a bit why there seems to be no universal CANBUS decoders available. What I mean is that the CANBUS interface itself is standardized (yes there are hi- and low-speed variants but it's easy to support them both). So a universal box would require some simple microcontroller which would allow it to be programmed for any available car in the market. It would just require a mapping table for car-specific CANBUS events to be translated into MTCD-specific events and in the other direction too.

Then it would just require a pretty simple software to edit those mappings so users could share their mappings. As a result we would quickly have a database with hundreds of car models supported. A bit like programmable universal remotes (e.g. Logitech) devices which can be software-configured for any TV/multimedia-equipment in the world.

Even a simple CANBUS dump feature to catch "unknown" events could be included. Allowing users to easily connect the universal canbus, press some buttons, hook it up to your PC and simply map the events generated to MTCD events. And then sharing the mapping with others.

Anyway... this is getting a bit off-topic.