It sounds like the ABS sensor has failed, its part of the hub assembly so you'll need to change the hub out. You can check the connector going to the hub for corrosion and/or looseness but in the majority of cases its the sensor itself that has failed. Rear hubs for ABS equipped HHR's can be found for between $130-160 US, they'll be a bit higher in Canada I imagine.
We won it, we won it, we won it!!! RDE 2/15/98
"Steam engines don't answer back. You can belt them with a hammer and they say nowt." - Fred Dibnah MBE (April 28, 1938- November 6, 2004)