Rusted wrench

I'm working on my 1997 Impreza Sport w/a 2.2, A/T. I've cleared numerous code problems by changing sensors, etc.. This car was sitting off the road for SEVERAL years. I'm down now to repeated codes of P0500 and P1540 vehicle speed sensor problems. Speedo works but reading live data while driving veh mph reading remains zero no matter what speed. From what I've researched on AllData, there are 2 sensors, one on the trans and one in the speedo head cluster. Unless I'm looking in the wrong place, the one I found on the trans, rear upper pass side corner, is connected to a wire that runs right into the vehicle harness and there's no plug to disconnect for replacement. OK, that's where I'm at, any suggestions to correct this problem? Thanks in advance for ANY help.