I have a III+ and a quest. For my 80 I got a RAM ball mount that is used to bolt onto like boat railing or round tubing. I got a u bolt that fit the mounting base, slid vacuum tubing/hose over the u bolt, and then mounted the u bolt/RAM ball mount on the rear view mirror stem up toward the top of the windshield. The mount has been there for some time now, and I have never had any problems with it it slipping or vibrating. I ran the power and antenna cables over the sunvisors and down the A pillar plastic trim and under the dash.
Depnding on what GPS you have, this may not be a good solution, but for smaller GPS units, I think it works pretty well.