I would but it's a company van that will be traded in for a new one in a couple of years.

So?

Do a non-destructive amplifier installation (put it under the passenger seat use velcro and duct tape if you want) and don't break any panels when you are stringing the wires. Replace the factory speakers with good ones. Put a good head unit in the dash. Keep all the old parts. When trade-in time comes, take the amp back out, leave the wires, and put the original radio and speakers back in. Use the amp and the good head unit and speakers the same way on the next company van.

It is not only possible, but quite easy to do a "temporary" high-end stereo installation.

tanstaafl.