Assuming you've already edited the base traffic packs as per the linked instructions, to also get them to appear on parking spots when used together with this mod you may need to edit the actual vehicle files. Not all mod authors disallow parked vehicles, but for example Jazzycat/Trafficmaniac does tend to block vehicles by default.

Open the traffic pack base .scs or .zip file and extract the entire \def\vehicle\ai folder somewhere.
Using Notepad++ (or another text editor that can find and replace across multiple files at once), open one of the .sui or .sii files under \ai\ or a subfolder of it.

Press ctrl+h to open the Replace dialog and go to the 'Find in Files' tab.

Select 'Follow current doc.' and 'In all sub-folders'.

Click 'Replace in Files' and let it finish. Once done, close Notepad++ and put the entire \ai\ folder back in the .scs or .zip file, overwriting if asked.

You will need to repeat this step every update.