Are you talking about the door inserts on the newer VW Beetles? those are a real pain to recover.
That residue is like headliner foam that has disintegrated.
I have used 3M Adhesive Remover, Acetone / Alcohol mix, Laquer Thinner, it's hard to totally clean that off.
FWIW Coverlay makes a hard plastic panel that can be dyed and installed to fix those VW door panels.
They fit and look pretty good and are pretty easy to install.

Thanks Mike,
We have been using Coverlays for many years and never checked to see if they made a product for VW doors. I'm going to order a set today to keep in stock. Most people will probably want these covered which will be so much easier.
But, the BIG question is " what is the most efficient way to remove the residue on headliners and related trim on European cars? "

I clean the foam off then wipe them down with lacquer thinner or brake clean. Then i sand off old glue by hand. I just watched a video on YouTube of a guy down south that uses xylene but i haven't tried it yet. His channel is William Hall. https://youtu.be/aqtyI1jolO0

I haven't done those particular door panels before. But I had a similar nasty foam / glue situation on some BMW sun-visors recently. The best thing I found was to use a heat gun and scrape off the majority of the glue with a putty knife and then come back and hit it with some acetone. Hope that helps...