Remove the head unit. If you are unfamiliar with how to remove it, here's 
.
Fit the Stereo Wire Harness Kit(American International Part #TWH950) to the 2 harnesses hanging out of the dash. They should be perfect fits. Now you should have a good clump of wires hanging out of your dash. Here's where it gets difficult to explain due to everyone using different colored wires. The two harnesses are logically seperated by what the wires are used for. One harness is for the output to your speakers, and the other harness is for the power, dimmer, accessory, ground, etc. On your new car stereo, the speaker outputs need to be connected to the harness for the speakers, and the ground wire(usually black), the constant battery power(usually yellow), and the accessory(or switched) power(usually red) are then connected to the other wire harness, as speciffied in the Stereo Wire Harness Kit. Note that since you have two head units, you'll be twisting a lot of the power wires and grounds together.
Remove brackets from your stock double-din sized CD player. (Note that your stereo is NOT the double din sized CD/tape player, you may have to make your own using American International Part #TOYK975). With your Philipps screwdriver, remove the mounting brackets. There are four screws on each side, as pictured below.
If needed, you may need to invent your own bracket, you can do it by using the side brackets of the Dash Kit(American International Part #TOYK975). The yellowed portions of the following picture are the brackets that you'd use, and to make them work you'd just have to drill holes in the sides that correspond to the mounting holes of the stack of 2 din units.
Stack your two din sized units one on top of the other. Magically, the holes used for your stock stereo will line up with the holes on the sides of both of your head units. Schweeeet!
Check for fit and make sure all of your wiring is correct by plugging the harnesses to what's hanging out of the dash. The next picture shows that I got lucky again :) Notice the gap around the orgy of two head units. There's nothing you can buy that will fill that. And also note that since head units may have slightly different sized/shaped faces, a "perfectly fit" trim piece will have to be custom made!
Fill that stupid gap! Here's where you need to get slightly creative. I used ABS sheeting, just because it's cheap, I had a bunch of it, and it's really easy to work with. I scored and snapped a few strips of the ABS, filed/sanded to size, and glued them together with acrylic glue. The ABS is flat, and the trim around the stereo is slightly curved, so I sanded the ABS to the shape of the dash.
I then glued on some small metal L-tabs about 1" long to the back of the ghetto trim piece that will pinch under the brackets to hold the trim piece securely in place. I used really thin metal strips that are bendable, meaning it could be slightly adjusted to fit inside of the dash trim piece perfectly. Below are pics from various trim pieces I've made. I've done quite a few.
FYI. Here's what my trim piece making "kit" looks like. Different colors and types of plastics and acrylics, plus the acrylic cement. Also -EVERY- workshop should have J-B WELD and that "QuickSteel" stuff, which looks like a tube of grey and black sushi that you slice up into cross sections as needed(and kneaded!)... mix the sushi slice with your fingers. Dries as strong as steel! That's what I used to attach the L-brackets to the back of the trim piece. This stuff is SUPER handy for fixing ANYTHING.
The result can be as perfect as you want... and if it isn't then you're totally in control to do it again. That's the best part of doing all of this yourself!
if you have any questions. Happy RAV'n!