GPS speedometer module location

[Che70velle]

For you guys that are using, or have installed for someone else, a GPS module for a speedometer, where did you place the actual antenna? Just purchased an Autometer GPS equipped unit, and the instructions tell me that the Antenna is waterproof, and can be installed on the roof (behind the headliner), under the dash, or perhaps even at rear package tray. I'm not sure about the lifespan of these antenna's, but I figure if I mount it behind the headliner, I'm asking for problems later? I've had a couple of cars factory equipped with GPS antenna's on the roof for navigation systems, so maybe it's not a bad idea?
Just wondering what some of you have done.