Windsor Transit runs service from Windsor to downtown Detroit, where you can connect to DDOT service to the Detroit Amtrak station. Not sure about the Dearborn station though.
EDIT: Just did a little googling, looks like DDOT Route 53 runs from Detroit Amtrak to the Rosa Parks Transit Center and DDOT Route 37 runs from (near) Dearborn Amtrak to the Rosa Parks Transit Center. You can pick up Transit Windsor Tunnel Bus at the Rosa Parks Transit Center. Check the online schedules to see if they fit the times you are looking for.