sloppz

Hello all i have a 1994 Subaru legacy and have fitted a 1998 Subaru wrx transmission. It all works well apart from my speed sensor the new wrx transmission comes with a three wire sensor, while the old body harness is a 2 wire one. Does anybody know of a way to get this to work? tried putting old one in and it fits fine but does not read anything. Thanks is advance. S