In my area the FM modulator does not work well. If I drive more then 10 miles I get interference. I live between 3 big cities so there is a lot of overlap. On a clear day I can pick up stations from all three towns that are 60 to 150 miles away. I use a cassette adaptor to hook up my mp3 player because of this.

This is the cheapest amp I could find without going to ebay.
http://www.cardomain.com/item/BOSCH150 $40

More than enough power for the front speakers. If you use it a full power you will probrobly blow the stock speakers. It might be small enough to fit in glove box. Or you might be able to mount it in the hole where the head unit was. Maybe you could fab a face plate that you could stick your mp3 player to with velcro for easy removal. You might be able to get a cover plate from a local installer.

You can get a cheap cable with the right connectors anywhere. All but the shortest ones should allow you to run the cable out of the glove box to the center console.

Bottom line is with the stock speakers it will not sound very good.