Here's something to try in the safety of your own home:
Go to "What fits my car" on Crutchfield.com.

Plug in 1991-Volvo-940-Station Wagon. Click CD players, and see all the items which say "Fits!"

Plug in 1991-Volvo-940 SE Turbo-Station Wagon. Click CD players and see all the items which say "Modified Fit".

I have a 1991 940 Turbo wagon. Not a 940 SE Turbo.

For the 940 SE Turbo, all Modified fit listings say:
» Caution! You will need to modify your vehicle to install this product. We strongly recommend contacting a Crutchfield Sales Advisor to discuss your installation and your options. Modifications may include cutting wires, plastic, or metal, and may include fabricating brackets, panels, or other parts, and the installation instructions or guides we may be including do not cover these modifications. Once your vehicle has been modified, it may not be possible to return it to the factory specs or appearance.
» You will need to fabricate a mounting kit in order to install your new receiver.

I called up Crutchfield, and they said "The regular 940 uses the part #120999200 for the physical front cover. A regular has a 2-inch tall DIN opening, the 940 SE Turbo has a 4-inch tall double-DIN."

I have a CR-814, which is about 2-1/8" high. The space in the dash, when I take out the built-in faceplate, is about 3-3/8" high. And if I take out that little shelf with the power socket on it, the space becomes about 5-7/8" high.

Crutchfield has always had very good advice for me on other vehicles. But they couldn't explain any difference by the missing letters "SE", so I came here. Are the non-turbo 940s like this?

And what has anyone else had to finagle out of sheet metal in order to get a DIN-style (non "two-knob") aftermarket stereo in a 940 Turbo? What fits, and what should I avoid?

Thanx,