how to put a tow hitch on a 2003 S7

I have a 2003 S7 which doesn't has a rear hitch on.  so just got a set of hitch from a 2004 G4 which comes with two big bolts for installation, but I was wondering how those two bolts would hold a big heavy hitch on to a truck.  am I still missing any parts of it?  Thanks!

Is this what you have http://www.roverparts.com/Parts/7475.cfm
 
or this http://www.roverparts.com/Parts/VUB104850G.cfm
 

The later one.  Thanks!

Hi, Sounds like your missing a few bolts etc, take a look here and all the info is there for you
 
http://www.discoweb.org/disco2trailerhitch/index.htm

So I was missing two 1/2 X 4 bolts, found them from HomeDepot and got the hitch installed.  Then I counted a question where to find the tow lights wires, I didn't see any wire plug underneath at the rear...

i dont know about your '03 but on my '99 the trailer light plug is behide the passenger side tail light.