An update - I took the car to a reputable car stereo shop that has done lots of work to my previous cars over the years so I know the owner pretty well. He said he would have a look at it and advise on what could be done, bearing in mind he had never heard of the Dynavin brand before.
What his installer confirmed was we need to run a cable from the boot to the front for the radio antenna to get working. Didn't specifically mention FAKRA but he pulled some cables off his shop shelf to show me. It didn't look like a FAKRA cable to me, just an antenna extension lead basically.
As for the steering wheel controls, they weren't able to help with that at all, and suggested I contact a Dynavin supplier for more help. You may be right that the Dynavin's harness has been cut off and spliced into what could be the stock wiring, which also doesn't have a harness. I don't know why it was done like that when a FAKRA cable should have simply made this a plug and play job.
I was quoted about $170 to route the antenna extension lead to at least get the radio working, but looks like nothing can be done about the steering wheel controls due to the apparent butchering of the factory harness. I didn't go ahead as I said I would need to think about it. $170 seems like alot for just radio reception, and still getting nowhere with steering wheel controls.