I had the same problem on my 2001 ls430, but mine was caused by a cracked lens and moisture accumulation behind it. Dealer wanted $1200 just for the part. I bought a used one from yahoo.jp for about $90 shipped to the states and it works fine. I had to take off the front bumper cover and the impact bar because the radar is mounted behind it, but it is definitely do-able yourself.
The only issue i have is it is calibrated for a right- hand drive car so sometimes it detects a car on my right during a curve and disengages, or downshifts to slow down, but if I shorten the radar distance it doesn't happen as much. I like to hang back from other cars to avoid rock chips.