[JosiahSpurr]

(...) Removing the door panels is quick and easy, you just need a screwdriver, and it should be very obvious if your linkage isn't hooked up right.

Careful with the forward tab of the map pocket when removing the inside door panel. It can break off. After removing everything and popping out the rivets around the edges, PULL the panel STRAIGHT up and not out because the tab from the map pocket slides down into this notch in the door:

 

[300Dman]

Are these rubber inserts available separately from the dealer or some other source?

I have not read that the inserts are available from the dealer. Look at some pics of the whole door stoker and determine if the rubber is the issue.

On my 84 W123 as an example what people have said they were doing is they were buying whole cheap door striker made by say URO or MTC and and removing the new rubber parts and putting them in to the original door striker.
The reason for the removal of only the rubber instead of simply using the cheapie door strikers is they seldom fit well or need to be fooled with to get them to fit. I bought the whole cheap door strikers for $16 each on ebay a few years ago and they indeed did not fit the same. It did not occur to me to only use the rubber parts alone.

There was a forum where someone said the bought only the rubber parts from Thailand. But, did not post a site to go through for that.

You are lucky your Door Striker Screws came loose as on mine and with others they stripped when I tried to get them loose. I have a thread on this form on how to remove them when that happens.