speedo meter not working in drive

I have a 1989 caddillac fleetwood RWD that the speedo works when I put the car in nuetural but while in any pulling gear d,1,2,or 3rd it stops working. I believe the cable to be good because it works when the trans is in nuetural. There is nothing touching the cable from the shifting rod.Any advice would be helpful. Thank you: James Verling