Any idea who makes such a cable, or where I could find it?

 

they used to be all over ebay.
a D2 is getting a little old, so you may have to look a little harder

 

If you go to the Ebay UK site, you will find a lot of special connections for these vehicles. Find what you are looking for and then search for supplier in North America or just order from UK.

 

I just bought a discovery 03 that had an iPod cable coming out of the glove box. I hope up an iPod to it and the iPod will charge, but how do I get the audio to play

 

That's a mod somebody who had the truck before you made. iPod had only come out 2 years before your truck was made (hard to believe).
You'll need to make an audio input if whoever did that didn't make one (but hunt around, someone who would go that far is likely to have).