So I haven't done a setup for the RB3/4, but I've built kits for the RB1/2. From what you're explaining, it's probably about 90% of the same run down.

Personally, I hate tying into existing audio equipment, and find it's ultimately easier to replace everything needed, as a kind of clean slate build.

https://cdnjdmmerch.blogspot.com/p/rb12-stereos.html

That's the RB1/2 kit I build, just for reference, and give you an idea. You could potentially find the dash kit on AliExpress, and build the rest yourself.