you got a multi-meter? if so you can measure the volts to the wiper motor with it engaged. but nay-say to that shite, if the blades screech then they are worn out, if they are new then your screen is prolly pitted. do yourself a favour and grab a buff with a nice gause pad, (i.e. the green stuff you wash dishes with) and run it on slow speed with a really coarse cutting compound. that will
hopefully get rid of ull the undulations*in your screen which will make the surface smoother
undulations IS a word, go look it up
if you have not got any of that stuff go pay a guy to do it for you.
*edit cos' of beer; yes my blades run slow all the time cos i have a mental voltage problem due to thermo fans/2xelectric fuel pumps/spastic stereo but i doubt yours is the same. you can measure the voltage at the wiper motor, but knowing there isnt enough volts at the engine is only the very beginning of the problem. seeking the answer usually means spending about 300 bucks on new electricals. and adding a relay wont do jack-all. also wiper motors are usually tough as f... so i would look at your glass before your motor.