On later cars (1978 onwards), the crossmember is being held by three bolts at each side, two at the rear and one at the front. These front ones are not the easiest to access, but they only need to be loosened a couple of turns because there are open slots at the front of the crossmember.
This type of crossmember has threads for rear seat belt buckles and these must also be removed (from inside). Even if you don't have rear seat belts fitted, there are two plastic threaded plugs - what laxeian is referring to on the above post - that still have to be removed. These are hidden under the luggage compartment mat, which needs to be removed first.