I wanted to do that forever on my 84, but the stock corner markers just aren't deep enough for an 1156 bulb. I did do it on my 90. I love it. The lights are twice as bright and actually shine towards the side as well as the front. Basically, I used some info I found on the internet and then did it a bit different. I drilled out a hole below the stock corner marker bulb hole, about 1" in diameter. Then got an 1157 socket (the 1156's didn't have a ground tab) with the spring tang ends, and jammed it in there. Then I wired it to the turn signal circuit. On the 84, both the corner and signal connectors were the same, so you could just swap them (I can't remember if they are on the 90). The only problem is that the corner bulb is smaller and less bright, and will also make the signals flash faster. That really isn't a problem, but the lack of light was the whole reason I moved the signals up there in the first place.
I actually followed these instuctions:
http://www.off-road.com/toyota/tech/turnsignal/index.html
