Firstly have you got a hole in you glovebox for the wire to go into?
Keep the headunit out. Get down in the passenger foot well on your back looking up under the dash you should be able to see the big whole where the headunit goes in. get you arm up there and poke the wire through and down into the footwell.
Now, keep in the same position, have a look for a hole on the side of the glove box. if you cant find one, you will need to take the dash top off. then you will be able to feed it through the glove box light hole.
When running the cable behind the dash just make sure it is secrure by using electrical tape, and make sure it isnt in the way of any moving parts.