View attachment 3837 AMX the little plastic cover you are mentioning is NOT a cosmetic item as you say. It has a very important task. It is only on the driver's side HT block. And yes it often gets bumped out of place & falls way down into never-never land.
The purpose of this little block is to balance the driver's side to the passenger side. The passenger side has a metal block with the wires to power the rear window defroster. Without the block on the driver's side the HT sits slightly, ever so slightly tipsy and this creates a rattling HT. So indeed if yours is missing you do need one.
May I suggest that when you get one you use some Super Glue to help hold it in place.
I know that these can be found
on eBay as well as Powersport/Thunderbird Specialities.