I don't know what the 'special' tool looks like, but I do know this is a very difficult job. I tried it unsuccessfully, then had a 'pro' and his helper try it and they could not get it in without damaging the trim. As a result, I found some trim that was plastic (looked like chrome)that had a wedge shape on the bottom, and it worked pretty easy. After a couple of years it started to change colors, but was still in place when I sold the car. So---the car I'm working on has a windshield with no groove for trim. I have several good pieces of the original trim both for the windshield and rear window if anyone needs a piece-or more. I do answer my phone. JIM

Here's my 2 cents worth.
A few years ago, I had a glass shop experienced in customs, street rods etc. replace my 544 windshield. They took out the seal with trim in place and put it back, telling me that the seal was in good condition. Shortly after, the seal started coming apart and they were willing to install a new one but would not tackle the trim.
After a lot of searching I found an out of town body shop willing to do the job. After 6 hours they gave up, having tried it with the the glass in and out, and badly damaging the trim, {which bends very easily.}
I ran an ad. on Craigs List: "Wanted - Old glass guy experienced with '60's Volvos" and found a retired glass guy who installed it in 20 minutes using the string method and an installer's tool with a twisted tip like a corkscrew.
Weeks later I found new trim to replace the battered original, and he agreed to do the replacement. It took 5 hours of sweating and cursing, trying it with the glass in and out! Eventually it worked best with the glass in in place.

Yes, it's called patience.

I believe the factory installed the trim after the glass was in place.
I was never able to do it that way for some reason. I ended up putting the trim on the gasket, the gasket on the glass and then installed the windshield. It wasn't easy.

It can be done with a lubed thick string/small rope and is really difficult, helps to have a second person around that doesn't mind random cussing.

Did you read the manual?
--
George Downs Bartlesville, Oklahoma