You just pinpointed the problem area,
Are there any non genuine reverse lights fitted, if so inspect the wiring to them and if no damage is found disconnect them and see if the short goes away.
If it is a manual disconnect the reverse light switch and see if it stops blowing, with ign on engine NOT running bridge the reverse light switch wires, if the lights come on but blows the fuse when the switch is re connected it needs a new switch, I have seen this several times in various Toyotas.
If the fuse blows when u bridge the switch wires, go looking for a damaged rear harness and look inside the light fittings for faults.
If it is an auto don't do the switch thing, do everything else then get professional help.
If you are not confident don't do anything, you can make it a lot worse, very quickly. Be sure the switch you are looking at is the right switch. With late model electronics, the damage is done before you see the spark, if you poke the wrong place.
Never put a heavier fuse in. So many times I have looked at melted wiring that is obvious that it had a very large fuse or piece of wire in the holder, but when I saw it, it had a fuse half the rated size in the holder.