Markus:
When you have any questions about a Kubota L Series Cab tractor, just come to me. Most guys here are familiar with my 5030HSTC. It's been posted numerous times....
First off, the Kubota radio is a Panasonic. I have the am-fm-weather band and the factory speakers are also Panasonic. They have plenty of Bass and the speakers mount right above your ears.
The antenna mounts on the mirror mount on the LHS of the cab facing forward. The Kubota antenna is a rubber ducky one that's designed to mount there.
The radio is a DIN mount, slide in chassis and I believe the rack is in the headpiece whether you get the radio or not. The loom is in all the cab models so it's just a matter of removing the roof, sliding in the radio and hooking the leads up. The speakers will attach to the radio. I think the whole setup for mine was about $250.00. Not that much when you consider the price of the tractor.