The provincial Highway Traffic Act prohibits having any object obstructing the driver's view through the windshield or windows.
Section 73(1) HTA states that "no person shall drive a motor vehicle upon a highway, with any sign, poster or other non-transparent material or object placed on the windshield or on any window of such motor vehicle; or with any object placed in, hung on or attached to the motor vehicle, in a manner that will obstruct the driver's view of the highway or any intersecting highway."
To avoid possibly running afoul of the law, it might be best to mount your GPS unit to the front of the dash (not on top) below the level of the windshield.
For safety, motorists should only rely on audio directions from the GPS rather than watching the screen when driving.
Charges under S. 73(1) HTA carry a $110 penalty. And, yes, before you write in, I'm aware that police themselves often have equipment (radar, large strobes etc.) mounted on their cruiser dashboard that blocks the driver's clear view of the road ahead. No exemption for police under this section of the HTA could be located.
Ontario Transportation Ministry :
Safe driving requires that you focus your attention on driving .
Navigation devices, when used properly, can make driving safer by helping to guide you to your destination with audio instructions as you drive. The risks associated with these devices typically occur when drivers try to manually reprogram them while driving.
Drivers who fail to pay attention to the driving task can face severe consequences.
For example, a "careless driving" conviction under S. 130 of the Highway Traffic Act carries a fine of up to $1,000, six demerit points, possible jail time, and a driver's licence suspension. Alternatively, a "dangerous driving" conviction under S. 249 of the Criminal Code of Canada carries a maximum $2,000 fine and up to five years in jail. The ministry will further examine this issue to see what next-generation driver distraction legislation could look like.