Recently my transmission (standard) on my 2000 gt stopped going in reverse so I purchased one off a 98 and installed it until I can get the other one fixed and the speedometer won't work can anyone tell me how to get it to work and/or what may be causing my old transmission to stop going into reverse

 

You need to swap the tailshaft section and plastic gear from your old transmission onto the new one for the speedometer to work.
96-98 t45 use a mechanical speedo signal (2 plastic gears meshing)
99-00 t45 use an electro signal , much like an ABS wheel sensor.
Easy to do, im doing one right now for my 96.