I have a new M7040 with the ultra grand cab. I just installed a radio so can offer some info.
I will paste 2 pages from the owners manual on the subject.
The support plate shown as 1 in the first illustration came with my tractor in the bag the manuals were in. It is the key to supporting the radio. If you dont have one you could easily make one out of very heavy sheet metal.
The mounting collar shown as 3 in the second illustration came with the radio.
I bought a Sony Radio CDXGT57UP for approximately $100 at Canadian Tire.
The support plate is slid into the opening and then the mounting collar slides thru the opening in the support plate. It is a snug fit. Once the mounting collar is in place you bend tabs on the collar out to lock the collar to the support plate. In my photo you can see the holes where the mounting collar tabs are bent out.
The radio comes with a wiring harness plug which fits the radio. Initially I asked the dealer if he could supply a harness to fit between the one on the tractor and the Sony radio. When he said it would be $52 and I would have to wait I decided to cut off the kubota plug and wire the new Sony radio wires directly to the Kubota wires. It was not hard. There are seven wires in the Kubota plug. Their color markings for the two speakers match the Sony wiring very closely. There are two wires for each speaker so now you only have 3 wires to figure out. One is a ground connected to the Kubota ground wire which is black. Now there are two wires left. Both are power wires. One is on all the time and maintains the memory in the radio such as preselected stations. the other is only on when the key is in accessory or on position.
I looked on line for plugs but really there is nothing which does not require some matching of the wires so I decided to just solder them together.
The whole installation was easy. The 2 speakers that came with the tractor sound good to me.
If you need more info just ask.
Dave M7040