OK, here’s what I did. First, my spider is a 1980 model; earlier cars may have different trim details, I don’t know. Also, I don’t claim that this is the best possible install … there are a few things I’m not 100% happy with.

Behind the inside rear quarter panel trim pieces there is a pocket with a drain hole and pipe at the bottom. When the top in down, the window sits inside the pocket. And when the top is up, water leaking in will tend to go down into the pocket.

This pocket is over the part of the rear wheel well where the rollbar needs to be fitted, so obviously some of it has to be cut away. Also, even to get the roll bar in, I had to cut away a good part of the pocket:

Note that I attached the top hold-down strap to the rollbar. The reason for this is discussed below.

It may be that if you take the pocket out completely, then reinstall once the bar is in place, you wouldn’t have to cut away as much material. But to do this, the bracket holding the top folding mechanism would have to be removed, along with a few other bits and pieces, and I didn’t feel like doing this. In retrospect, this was maybe a mistake.

Anyway, the next problem I hit was that the quarter panel trim would no longer fit. The IAP instructions talk airily about trimming this panel, but as far as I can see, some fairly major surgery is needed. After fiddling with cardboard templates, etc., etc., I ended up cutting as shown:

The other panel, uncut, is shown for comparison. A PO had installed speakers in these panels, so I didn’t feel too bad hacking at them. I might have felt differently if they were pristine.

Once the panels were cut, the vinyl coverings started to come away, and since I needed to cover the speaker holes anyway, I recovered with black vinyl that I happened to have lying around.

The picture at right shows why I think the panels need to be cut in this way. Basically the rollbar interferes with the panel over its entire height:

The back seat also has to be hacked away to fit around the bottom of the rollbar. To reattach the folded-under seat cover, you really need a few hog rings, applied with the correct hog ring pliers:

This actually doesn’t look too bad once installed

After the quarter panel surgery, there’s nowhere on the panel to attach the top hold-down strap. It can be fixed to the rollbar, where it works just fine.	This also provides a place to attach the top hold-down hooks:Not perfect, but acceptable, at least to me.
Finally, I need to move the seat-belt guides along the top of the seat, since the belt now wants to go through the middle of the rollbar: Overall, I’m OK with the outcome. But it was a lot more fiddly than I expected. And I might have second thoughts about doing it again. Hope this helps.