The motor doesn't spin forward and backwards, it only spins one direction. The arm linkages transfer that rotation in moving the arms backwards and forwards across the screen... it doesn't matter which direction the motor is bolted on at...
The only critical part is the motor is at the "park" position of rotation (it has a sensor inside the motor itself). If you bolt it up say at the half way point then the wipers will work normally on low and high speed but when you turn them off the may not be at the bottom of the screen... they might be half way up or whatever.
So in the past I've wired them up, let the motor cycle through and turn it off. After the motor stops I move the wipers to the bottom of the screen and then attach it to the spline...
If you get it wrong it's easy to readjust.