That's the line you're looking for:
oPartsList.Style = oDrawDoc.StylesManager.PartsListStyles.Item("PartsList(nameofthepartlist)")
Here's the rule i use. Of course you'll have to tweak it a little bit (name of partlist):
Dim openDoc As Document
openDoc = ThisDoc.Document
'Dim docFile As Document
If openDoc.DocumentType = 12292 Then
'Look For the model referenced within the drawing. End the Rule If the drawing Is empty.
Dim MDocFile As Document
If ThisDoc.ModelDocument IsNot Nothing Then
MDocFile = ThisDoc.ModelDocument
Else
MessageBox.Show("It is forbiden to perform this action with an empty drawing", "Export error")
Return
End If
'assumes a drawing document is active.
Dim oDrawDoc As DrawingDocument
oDrawDoc = ThisApplication.ActiveDocument
'Set a reference to the active sheet.
Dim oSheet As Sheet
oSheet = oDrawDoc.ActiveSheet
'Look for partlist within drawing. End rule, if it doesn't exist.
'say there is a Partslist on the sheet.
Dim oPartslist As PartsList
oPartslist = oSheet.PartsLists(1)
If oSheet.PartsLists(1) IsNot Nothing Then
'set parts list to a specific style
oPartsList.Style = oDrawDoc.StylesManager.PartsListStyles.Item("PartsList(nameofthepartlist2)")
End If
Else
MessageBox.Show("You must have a valid Drawing document open before using this code!", "File Type Mismatch!")
End If