having 2 head units

i just bought a 2005 hyundai elantra, and it comes with a cd player, and i already have a Minidisc head unit.

does anyone know if there is such thing as an external case to place my MD player in? or any other ideas?

any advice would be greatly appreciated!

Judging by the title of the post, I'm assuming your MD player is a head unit, and not a portable.  Either way, you may need to build something. 

To house it, you might fit it in the console, or the glove box.  You could probably use this: http://www.metraonline.com/product.aspx?sku=99-9000 but it would be ugly.  Your best bet is to come up with some bracketry to secure it in place, then make trim panel to finish it off and make it look nice.

What we've done in the past, is use a special relay to switch the speakers when you turn on the secondary unit.  Tie the negatives together, and switch the positive leads.  Use the remote turn on to switch the relay.

Search Radio Shack or other electronic stores for a 4-pole double throw relay.  I'm not sure where we got ours, but it sure beats having 4 Bosch type relays.

Just wire it like above- connect the negative speaker leads of the decks together.  Wire the stock unit's positive outputs to the NC contacts, and the MD positives to the NO contacts.  Connect the actual speaker positives to the common posts.  Use the MD remote turn on to trigger the relay.