I have modified many VDO Gauges and its quite easy to get in to one. the first thing i do is take a small common screwstick and insert it between the gauge body and the bezzel. then lightly pry/ dig under the bezzel. go around the whole body opening the bezzel. once you open it do the modifications you wish then place the bezzel back and gently push/hammer the bezel back down and your ready to rock and roll.
more info can be found at
http://www.pelicanparts.com/techarti...uge_repair.htm