Spec's for 99' Montero/Infinity sound?

Just make sure you get something that has the correct impedence and is the right size.  And unfortunately that can vary widely among similar vehicles and the only way to make sure you know the right impedence is by doing a resistance check on the speaker itself.  Or of course if it says on the back of the magnet the ohms then that'll work too.  And remember if you happen to look at component midranges make sure it's not one made for home use cuz those are usually all 8-ohm speakers.

You can probably get something sounding similar to the one that's in your vehicle from Parts Express or another electronics catalog type place.  Of course you can also get some from Crtutchfield or any regular car audio retailer, but they'll only sell you them in pairs, whereas the electronics catalogs let you buy single speakers.