The Bureau of Broadcast Measurement provides data on radio station market share data for Canadian cities on their web site here.
The most recent survey only covers the period Jan. 8 - Mar. 4, so it does not reflect the recent programming changes and the effect it has had on CBC Radio Two's market share. The next survey, it appears, will be available mid-June. You can use this data to track CBC Radio Two's market share in your city and see for yourself the impact of the recent CBC Radio Two programming changes!