How do I add a Yahoo search box to my site?

Dave's Answer:
I agree with you about how complex Yahoo's search code is. In fact, I think that deciphering the Yahoo home page source code could easily be the final exam of a graduate level computer science course. It's really unbelievably baffling!
Fortunately you can reverse engineer the search by simply looking at the URL that is produced when you do a Yahoo search, strip out the unnecessary variables, and then build a rudimentary HTML form that produces the remaining. Let me show you...

I'll do a Yahoo search for Microsoft Zune and here's the URL that's produced:

http://search.yahoo.com/search?p=microsoft+zune&fr=yfp-t-501&toggle=1&cop=mss&ei=UTF-8

Now, within the Web browser's Address box, I'm going to experimentally remove specific name=value pairs to pare it down to the bare minimum. Turns out that the following much simpler URL produces the exact same search result:

http://search.yahoo.com/search?p=microsoft+zune

Nice! You can avoid all the other mumbo-jumbo, and thank goodness.
The form that produces that search looks like this:

 

That's all there is to it. Here's a Yahoo search box that has exactly that code underneath it:

 

One more tweak, though, just because that search box looks just a wee bit boring! You can drop in some minimal CSS and put a nice little box around the search form:

style="border:3px double black;padding:5px;">

 

Which now looks like this:

 

Hopefully that'll help you out!