1999 Toyota Sienna amp question

OEM systems require specialized connectors and factory installed wiring that may or may not already be in your vehicle.  Find the factory mounting locations for the components and see if there is an unused connector in the general area that mates with the equipment.  If not, you will have to build your own interconnects.  Get the factory wiring diagrams from your dealer to see what the pin-out is supposed to be if you have to build your own.  This is difficult to make work correctly.

What you are attempting is an expensive, time-consuming proposition that may end up in failure if you cannot determine or obtain the proper connectors.  Installing an after-market HU and CD changer is a far wiser and easier route (and maybe even cheaper, as OEM stereo equipment is often WAY overpriced.)

The PAC-SWI-X allows an aftermarket cd player (which would be way better then your OEM one ,be alot less expensive ,and sound better). Just buy this along with a cheap easy to use aftermarket receiver that come with a wireless remote. A HU like this Clarion one would be a good choice because its easy to use: http://www.crutchfield.com/cgi-bin/S-lohrX8fyGrz/ProdView.asp?s=0&c=3&g=300&I=020DB245&o=m&a=1&cc=01&avf=Y 

You can find all of these things at crutchfield.com. Just shop from this link which already has your vehicle information selected: http://www.crutchfield.com/cgi-bin/S-lohrX8fyGrz/autoinfo/autoinfo.asp?s=0&cc=01&vehicleid=11706 They give you excellent instructions and free tech support so I think this is one of the best places to shop for car audio online.