Which one do you have? Mine is mounted with brackets that sit in a sort of recess under the carpet directly beneath where the parcel shelf clips in. Then threaded sockets poke through a little hole in the carpet and the bar threads into these to tighten up. See pics in my readers ride thread (link below).
 
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This is the Hugh's Engineering one that I think BoomhaueR is talking about
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These have plates that sit inside the recess'
25072009337.jpg


Looking at the Ebay one you will need to drill hole for the bolts just below the recess to mount the brackets