Test the reversing switch using a multimeter for continuity as shown in the attached picture. With the switch set as if it has completed the "up" cycle test between R and orange, then test between L and blue. Next flip the switch as if it has completed the down cycle. Test between the R and blue, then L and white. If you have an open in any of the tests you've found that problem. GM discontinued it long ago, but I sell it inder my part number 205180 on the web site www.top-downsolutions.com.
