There is a PDF showing how to do this, click on the Index of .... shown in red in my signature you.
Go to the BODY section and look for the PDF on how to add the missing Isolator.
You can do it with the cowl still in place or take the cowl off and do it. I suggest taking the cowl off and clean the area underneath it to remove debris that can plug the drains and cause you serious problems later when the drains become plugged and water overflows into the heating air vent.
You can run thin wires down the drains to make sure they are free of debris now.