What amperage wire do I need for wiring in a windscreen wiper motor?

not sure on the current draw of the motor, but as most wipers have a fuse in line, the wire must be capable of handling at least the fuse current. If you have a 15Amp fuse then in my mind this needs to be the weak link not the cable.

hope that helps

Cross sectional area, in millimetes, is the measurement typically used to size wire.

1.5mm single core multi strand should cover it. As previously advised, fuse the circuit to cover the motor current draw. Typically 15A will do for a wiper motor.