I fixed sat nav on using ram mount fixings,where the clocks are bolted on,remove bolt and replace with ram mount ball fixing ( Aluminum 1" Ball on 1/4 NPT Female Thread RAM Part: RAM-B-218-1)
then attach ram mount arm to ball ( RAM U-Bolt Mount (Aluminum) RAM Part: RAM-B-149 )
fix sat nav holder to bracket
you can find all the bits on ram mount site, this is the uk site,but there will be a site in your country i'm sure
RAM Mounting Systems UK - RAM Mounting Brackets
hope this is of some help
andy