I agree that this is more of a discipline. Maybe you can motivate the team by explaining to them the short term and long term benefit of unit testing. Site good examples if you can. Do pair programming with them and lead by example.
Or if not, enforce it to the team
. What you can do is to require them to have unit test where it make sense as part of your code review process. Then that will become a practice in their day to day activities. Or as previously mentioned, establish a metric regarding code quality to measure if you are improving or not in that aspect and make it as part of your team goal.