Funny you should ask...This is the first mod I performed on my SVT when I bought it. That was a year ago let me see if I can remember how to do this!
Under the dash, there is a box attached to the fuse box. It sort of unplugs, which I did.
Then I took a pair if wire cutters and cut a hole in the side of the box near where the speaker is. Be carefull where your cutting.
Then I took a knife and cute the two leads on the bottom of the little speaker. By the way the speaker is a circular black plastic thingy (It doesn't look like a real speaker, almost like a speaker on a modem only biger)
Anyways once I cut the speaker free I removed it completely and took some electrical tape and tried my best to cover the two now cut leads. Then once you are doen plug it back in and TADAH no sound.
This is just what I did becuase I couldn't figure out what else to do! There may be a less painful way. If you would like more explaination let me know, I might be able to elaborate more.
Sound ghetto but it worked for me!
Hope it helps.
Justin