Adding custom JavaScript to an Out Of Box Ribbon Button

Hi Guys,

I am trying to add a JavaScript function to the out of the box ribbon button 'Add Existing". Below is what i currently have, this does not seem to be getting the job done. Any ideas as to what I may be doing wrong? 

<RibbonDiffXml><CustomActions><HideCustomAction Location="Mscrm.Form.EntityName.SaveAndNew" HideActionId="Mscrm.Form.EntityName.SaveAndNew.HideAction" /><HideCustomAction Location="Mscrm.SubGrid.EntityName.AddNewStandard" HideActionId="Mscrm.SubGrid.EntityName.AddNewStandard.HideAction" /><HideCustomAction Location="Mscrm.SubGrid.EntityName.Edit" HideActionId="Mscrm.SubGrid.EntityName.Edit.HideAction" /><HideCustomAction Location="Mscrm.SubGrid.EntityName.NewRecord" HideActionId="Mscrm.SubGrid.EntityName.NewRecord.HideAction" /><CustomAction Id="Mscrm.SubGrid.EntityName.AddExistingAssoc_CustomAction" Location="Mscrm.SubGrid.EntityName.AddExistingAssoc" Sequence="40"> <CommandUIDefinition><Button Id="Mscrm.SubGrid.EntityName.AddExistingAssoc"  
			Command="Mscrm.AddExistingRecordFromSubGridAssociated_CustomAction"  
			Sequence="40" LabelText="$Resources(EntityDisplayName):Ribbon.SubGrid.AddExisting" 
			Alt="$Resources(EntityDisplayName):Ribbon.SubGrid.AddExisting" 
			Image16by16="/_imgs/ribbon/AddExistingStandard_16.png" 
			Image32by32="/_imgs/ribbon/AddExistingStandard_32.png" 
			TemplateAlias="o1" 
			ToolTipTitle="$Resources(EntityDisplayName):Mscrm_SubGrid_EntityLogicalName_MainTab_Management_AddExistingAssoc_ToolTipTitle" 
			ToolTipDescription="$Resources(EntityDisplayName):Mscrm_SubGrid_EntityLogicalName_MainTab_Management_AddExistingAssoc_ToolTipDescription"/></CommandUIDefinition></CustomAction></CustomActions><Templates><RibbonTemplates Id="Mscrm.Templates"></RibbonTemplates></Templates><CommandDefinitions><CommandDefinition Id="Mscrm.AddExistingRecordFromSubGridAssociated_CustomAction"><EnableRules><EnableRule Id="Mscrm.AddExistingRecordFromSubGridAssociated_CustomRule" /><EnableRule Id="Mscrm.AppendToPrimary" /><EnableRule Id="Mscrm.EntityFormIsEnabled" />				</EnableRules><DisplayRules><DisplayRule Id="Mscrm.AddExisting" /><DisplayRule Id="Mscrm.ShowForManyToManyGrids" /><DisplayRule Id="Mscrm.AppendToPrimary" /><DisplayRule Id="Mscrm.AppendSelected" /></DisplayRules><Actions />			  </CommandDefinition></CommandDefinitions><RuleDefinitions><TabDisplayRules /><DisplayRules /><EnableRules><EnableRule Id="Mscrm.AddExistingRecordFromSubGridAssociated_CustomRule"><CustomRule FunctionName="myFunction" Library="$webresource:MyFile" Default="false" /></EnableRule></EnableRules></RuleDefinitions><LocLabels /></RibbonDiffXml>

Thanks