You need to trace the cable really, it cant have gone far, then when you get it you need to make it taught round something, you could use the broken bonnet catch from inside the car and you need to pull it, there doesnt need to be much play in it if the cable is tight enough, it can take a few goes but it should work. Are you sure the catch is broken and not just unclipped from it's housing ? Sometimes the cables can get a little stretched and you need to feed them back through a bit in order to make them tight. I don't know the setup on a mk3 I'm just going on past experience with other cars but the principle must be the same. If this faills and you cant find the cable then pass, I guess it's taking a bit of the interior panels off in order to fine the damn thing.
