When asked what an employer should do when it discovers that T4 slips issued by it incorrectly showed an automobile benefit, CRA stated:
[S]ubsection 152(4.2) … covers, in particular, situations in which an employee, who has received T4 slips that have been amended for previous taxation years, requests the CRA to correct the income tax return already filed for those taxation years. In your situation, the employer could therefore prepare amended T4 slips for the previous 10 calendar years.
Where an employee wishes to obtain an adjustment to an income tax return already filed for a taxation year that falls within the normal reassessment period, the employee should request the CRA to correct the employee’s income tax return by filing a duly completed Form T1-ADJ, T1 Adjustment Request, or a signed letter that provides all the information regarding the employee’s request, to the employee’s Tax Centre. If the relief the employee wishes to claim is for a particular taxation year that is beyond the normal reassessment period, the employee will instead be required to file a duly completed Form RC4288, Request for Taxpayer Relief at the employee’s Tax Centre.