I'd start by checking the basics - take out the boot shelf if it has one, to see if there's a courtesy light not going off.
Next I'd try disconnecting the stereo.
Start by pulling a few fuses for "courtesy" items like the above, maybe the cig lighter, alarm and so on.
It doesn't cost anything and may save you some pennies if you can pinpoint the fault.
Have you had a look on the Rav car forums to see if anyone else has had the same problem?