If its like mine, open the boot, and look for a metal box to the rhs of the spare wheel, thats the dvd/cd sat nav unit. Unbolt the guard and there's an eject button on the front (facing the spare), out it pops and pop in the new one. Lucky you by the way my nav won't work as that unit is u/s, £150 to mend apparently so its mr tomtom or his mates for me, still I can amuse myself by watching the screen go up and down anyway!Must put a request on the wants board!
You could get a DVD GPS double DIN headunit for about £200. I'm looking at getting one myself but I've never heard of the 2 companies that make them cheap! If interested try eonon.com or xtrons.com B)