The subject of Police powers regarding traffic stops is thoroughly covered here:
http://www.canlii.ca/eliisa/highlight.d ... 23355.html
Sadly, it appears Bear is correct with respect to our pathetic lack of civil rights when it comes to being stopped. The police CAN stop anyone, anytime, for no other reason other than to check Lisence, Insurance, and mechanical condition of the vehicle. There is a section that deals with R.I.D.E.-type programs that highlights the fact that police must not exceed their authority by investigating matters BEYOND these particular issues. Perhaps you could argue that your eventual ticket was for something outside the intended scope the R.I.D.E. program which is considered an acceptable inconvenience for the purpose of getting drunks off the road and making the highways safer for responsible drivers such as yourself 
"Check stop programs result in the arbitrary detention of motorists. The programs are justified as a means aimed at reducing the terrible toll of death and injuries so often occasioned by impaired drivers or by dangerous vehicles. The primary aim of the program is thus to check for sobriety, licenses, ownership, insurance and the mechanical fitness of cars. The police use of check stops should not be extended beyond these aims. Random stop programs must not be turned into a means of conducting either an unfounded general inquisition or an unreasonable search...."
In this case, Lisence, Ownership and Insurance were provided, and no mechanical defects were noted. No "safety" concerns resulted from this stop, but the stop transgressed in to NON-safety related statute violation. Sure it's a long-shot, but I'D try it, if I were me
. Your problem is going to be that the plate is in "plain view". So I'd only try this weak defense if the Crown was not willing to offer a decent plea deal.
"The detention authorized by s. 216(1) of the Highway Traffic Act is circumscribed by its purpose. The detention is limited to the roadside and must be brief, unless other grounds are established for a further detention. The police may require production of the documents which drivers are required to have with them and may detain the vehicle and its occupants while those documents are checked against information available through the computer terminal in the police vehicle. The police may also assess the mechanical fitness of the vehicle, examine equipment for compliance with safety standards and from outside of the vehicle, make a visual examination of the interior to ensure their own safely in the course of the detention: R. v. Ladouceur, supra at 1286-87; R. v. Mellenthin, 1992 CanLII 50 (S.C.C.), [1992] 3 S.C.R. 615 at 623-24, 76 C.C.C. (3d) 481; R. v. E. (G.A.) (reflex-logo) reflex, (1992), 77 C.C.C. (3d) 60 (Ont. C.A.). More intrusive examinations or inquiries directed at matters not relevant to highway safety concerns are not authorized by s. 216(1) of the H.T.A.: R. v. Mellenthin, supra."