I've got a writeup on the Toyota MR2 Owners Club website with installation details for a Pioneer AVIC D3 in a 1992 Toyota MR2 turbo. Includes a picture of the location of the VSS wire and follow-ups by other posters.

 

Check it out here: http://mr2oc.com/showthread.php?t=258032

 

Short summary: The VSS wire can be found leading to the cruise control computer behind the glovebox. It is a purple wire leading to pin #20 in the connector. The glovebox itself can be removed by unscrewing 5 screws surrounding the box. Two screws are inside, behind the door. Three screws are outside, at the bottom edge of the glovebox (two on the left, one on the right). There is also one clip holding the box (just pull on the box and it will release).

 

You need no additional installation components, as the AVIC D3 fits well in the double Din opening, and is held in place by the factory brackets. You can splice directly into the stock harness or buy the appropriate Metra harness (varies by model year).