What is interesting is that, unlike Mexico, none of Canada's larger cities, though they are largely in the southern parts of their respective provinces, are close enough to any U.S. cities of any significant size where U.S. radio stations could greatly influence the ratings of Canadian stations. The closest Canadian city to any major U.S. city (and this includes both Buffalo and Bellingham) would be Windsor, Ontario, which is just across a short strait from Detroit. I'm sure both Fybush and Huff will correct me if I'm wrong (and please do) but I wouldn't be surprised if the Detroit radio stations dominated Windsor radio ratings now in the same way that CKLW dominated Detroit ratings back in the 1970s.
The only other U.S.-Canadian markets that I think would be close enough where the radio stations could influence the other side of the border are also along the Michigan-Ontario border and are a lot smaller. They would be Sault Ste. Marie (both Michigan and Ontario) and Port Huron, Michigan/Sarnia, Ontario which is an hour or so northeast of Detroit.