Well, you're close! The nut in the center is not a ground but for fitting the radio in a bracket (example below on a slightly different model, but not mandatory). The brown wire is negative/ground and the way it is hooked up appears to be ok, purple is plus and also ok by the looks of it. Makes me wonder where the yellow and green chassis wire goes! A second ground connection? The big black connector is a din connector that can indeed add an auxiliary input.
Loudspeakers: most of these models have two loudspeaker outputs, another one next to the one that is currently being used, pls check. It appears that one speaker output is now being split for 2 speakers, pls check wiring, also behind the speakers, and make sure the speakers are not driven through ground but are using the two wires from the radio.
If the radio has traffic info (VF/DK/SDK) pls check if its not switched on, either separate button or by pulling out the colume control knob.
Antenna: you defo need to have the antenna extended for a decent reception and if that does not do the trick it may be caused by bad wiring, or by a bad center pin connection inside the radio (if you have a very small screwdriver you can try to pry it a bit making the connection a bit more tight.
Bottom line: I've worked with many of these radio's and they're pretty much bomb proof, have pretty powerful amps and FM reception usually is pretty strong so in first instance I'd look into the wiring.
Another model I had on my bench recently looks pretty similar, is missing the separate ground connection but has this located right next to the +. Both should work in a similar way.
