I did this on my wife's 04 Highlander right after we got it. The following will supply power to the front lower console and rear outlets, Not the cigarette lighter just below the radio. We use it to charge Cell phones, Ipods and gameboys when we travel. We have yet to run the battery down.
Also this was for the 04, don't know if the earlier or later models are different as Toyota even makes mid-year changes.
The attached .pdf you will find a picture of the REAR of the interior fuse panel. This is difficult to get to as it tucked right above the emergency brake. Remove the ACC relay. This is the closest one to the kick panel ( the other 2 were smaller and blue in color ). Also the largest of the 3 on the bottom. It doesn't come out easily as it's held by 2 plastic tabs that must be released on the sides by a small screwdriver.
Go to Radio shack, or anyplace to get a standard "bosch type" 12v 30a relay. Remove the top of the relay and remove the relay itself. Then solder a wire from points 30 to 87 of the relay base ( effectively "closing" what was the relay points. BE CERTAIN POINTS 85 & 86 ARE NOT TOUCHING ANYTHING ELSE IN THE RELAY. THEY WILL NOT BE CONNECTED TO ANYTHING INSIDE ANYMORE. Reglue the top on the relay and install it in the ACC relay position and Voila, Acc outlets always on.
You can also accomplish this if you make a simple jumper to jump from points 3 to 5 in the relay base where the ACC relay was. But this was a difficult enough place to work in to begin with, so I went with the Alter a Relay method.