I had the same problem once on another vehicle. Turned out to be the switch on the column. Unfortunately, the dimmer switch was part of the entire steering column control cluster, so I had to replace the whole thing, at a cost of $100 on ebay. I have not yet torn into the Toyota steering column yet, so I'm not sure what you'd have to replace. Maybe some one else can offer some insight into that. I would be willing to bet that the same switch is used on pratically every truck/4runner of your year range.
