When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I would try to find a complete tailgate at a junk yard if it were me. The circuits for the back window are complicated, having the switch in the dash providing grounds for the tailgate switch. That allows the motor to run each way from either switch. Also there is a torsion spring inside the tailgate that helps raise & lower the gate that is likely loose if that much is rusted out. If not loose now it could snap loose and break the glass when it is lowered. There is a replacement panel for the bottom of the 'gate if you can't find a better one. I think broncograveyard.com carries them, and probably LMC truck or Driven Auto Parts have them. Or you can go with big bucks and get a whole new tailgate shell.