Now selling again
Sold in sets of 5 (all are 10mm in size now), in case one ever breaks.
You'll basically only need 1, to wedge between the left side of the dash and the windscreen glass. But some people had good results with putting another on the right side of the dash.
Usually 20-25cm from the side is a good spot, but the location doesn't really matter. It depends more on where it'll fit, because some people have a bigger/smaller gap between their windscreen and dash. That gap can vary from the side to the centre (can be due to the differences in how well a windscreen was replaced).
*Tying a piece of string through the DRS helps if you accidentally drop it (which is easy to do). When the DRS is securely in place, simply cut the string.*
Thanks to bigbadbenny for mentioning that a US Legacygt.com forum member makes these 3D Printed Dash Rattle stoppers. As they say "The dash rattle stopper is a rather inelegant solution but it works. It is a small piece of plastic that gets crammed between dash and window just to the right of the VIN tag". Is not visible once installed, and feedback has been great - it really works!
http://legacygt.com/forums/showthread.php/3d-printing-development-thread-231380p11.html