Vande,
it depends where you want to mount the retractor. In one version - preferred by a very knowledgeable German vendor (Pagodenteile.de) who supplies bolts and a counterplate that you can access from underneath the car. No welding is required and only two small and one bigger hole need to be drilled. Shown in the first attachment with German instructions. It is a very easy installation., but it is a bit tricky getting the retractor spool to be sitting parallel to the floor otherwise the lock will not work!
JA17 uses a different method shown in the second attachment. It has the advantage that you have more room for the spool.