The wires themselves will not appear to be worn or broken in any way, but for some reason they are shorting out somewhere (assuming the sensors are ok, which according to the supras.com write up they almost always are).
My check engine light would come on after a few minutes of driving when I had the problem.
Have you seen the write up at
http://www.supracentral.com/oldschool/htm/knock.htm
(I'm sure you have seen this).
I would bet it's faulty wiring, and since it only costs about $15 in wire/solder, it doesn't hurt. It took me and a buddy about an hour and a half, I was scared to touch the ecu but after we started digging around we realized how easy it was.
New knock sensors are something ridiculous like $200 each, so definitely try the rewire first. Do you have a pre or post 89 supra? Aparently with the post 89's it's hard to get to the second sensor, but some people say they can get it. With my 88 we did not have to remove the starter, although my buddy did have small and agile hands.
Good luck!