Vehicle Speed Sensor issues -- replacements not working.

Hello, I have 2002 Lexus IS300, standard transmission.  My speedometer was acting up, jumping all over the place, although the CEL came on only briefly, which was the P0500 code (Vehicle Speed Sensor).  My cruise control also doesn't work.  Now my speedo has just died, flat, not working, and the CEL P0500 is on again, of course.  However, I have now twice ordered a replacement Vehicle Speed Sensor (part code SC414) and neither brand new VSS has worked -- the speedo just stays dead.  What else could it be?  I thought the first replacement VSS might be defective, but, after the second one now, I have to think that something else is the problem, and maybe even my original VSS is just fine, but I've taken it to several shops and they don't know what else to look for and I really can't afford the dealership prices.  Any help would be just great.