This give you some ideas?
Code:
Private Sub Report_Open(Cancel As Integer)
Select Case Forms!frmReports.fraGrouping
Case 1 'sort/group on car type
Me.GroupLevel(0).ControlSource = "CarType"
Me.txtGroup.ControlSource = "CarDesc"
Case 2 'sort/group on company
Me.GroupLevel(0).ControlSource = "Company"
Me.txtGroup.ControlSource = "Company"
Case 3 'sort on date/time, no grouping
Me.GroupLevel(0).ControlSource = "DispDateTime"
Me.txtGroup.ControlSource = "DispDateTime"
Me.GroupHeader0.Visible = False
End Select
End Sub