It's difficult to tell exactly without actually being there. Do you have a picture of the black connector with the six holes?
The thick black one you've highlighted is the aerial.
The white/cream block in the picture is not one I'm familiar with to be honest.
The black one that didn't do anything when removed could be an earth but the radio is probably earthing another way, especially is its mounted using metal brackets and metal screws.
How many speakers do you have in there, and are they all standard landrover or aftermarket?
How confident are you with vehicle electrics? It does seem from your description that you've got an odd little collection of wires. But like I said, it's difficult to tell properly without being there.
If it were me I'd figure out what they all are, remove everything I didn't need, then solder on a standard ISO fitment which would make everything else in the future so much easier. If you don't fancy doing it yourself I reckon an auto electrician could do it for about £20-£25.
If you do decide to do it yourself and you think I can help, drop another post and I'll see what I can suggest.