First have you checked behind where the radio goes and see if the radio plugs are there? I ask for one time I bought a truck that did not come with the radio from the factory but had the wiring harness already in place. So you might find that the wiring is there and all you need is to add speakers and the radio. Most auto parts stores or Wal-Mart sell the harness adapters that you wire to the new radio and plug into the van's harness.

If no harness then look in the fuse panel for where the radio fuse went and plug one side of a fuse holder into it and run that to your radio then it will work with the key.

If you do not find the original fuse slot in the fuse panel box then look for a fuse that is only on with the accessory position of the key. Then pull the fuse and find the side of the fuse slot that is hot with the key on. I don't part number but there is a kit that allows you to add a terminal to the slot then put the fuse back in. At this point you connect the radio power wire to it and it only comes on with the key as the stock radio did.