I am about to buy a used Malibu TXi for slaloming in the Houston area. Based on the current listings, it will probably be a 2012, 2013, or 2014 model. Several of them do not have Zero-Off with GPS, so I would need to have it installed. Does anyone have any experience/advice adding Zero-off to an existing boat? Who did you get to install it and were you happy with the results? I understand that StarGazer could be added, but I prefer Zero-Off.