Day/Night Rearview Mirror Switch

This problem is not my CC's but my '70 98. I know some of you boxie wagon owners have the older mirrors with the sideways flip switch also. Mine refuses to stay in night mode, it immediately springs back to day. I can manually adjust it to the position by moving it but the switch will not hold it there. Has anyone ever had this problem? If so, how did you fix it? I don't want to replace my original map light mirror with a repro.

Thanks,
Steve