Hello, and welcome to Just Answer. My name isXXXXX can help you on this concern.
Items like the Speedometer and odometer are usually assembled in a way that if you try to disassemble it it breaks it, auto manufacturers do that to try to stop people from rolling back the odometer. The best option is to just get a complete assembly, you might be able to find one at a u pick it salvage yard. That would be my recommendation if you want a ven with a working odometer. If you need more info Please click "Reply"
I Have to say it's not very often that I disassemble a speedometer, but if theyre plastic melt rivets, or even metal rivets, I'd recommend getting a dremel and grind the heads of any rivets, that would make it much easier to separate the parts.