I have to agree with the statement that there are two different types of plastic involved. In my FX4 they are also 2 diff colors. I think one heats up faster than the other and causes a gap between the two. After the temp changes in the cab, the squeak goes away. I think this only started after I replaced my stereo head unit. Have to get back in there.

 

Check the wiper motor clips. Pull the leave screen off and check the bolts securing the wiper motor (U-Clips have a problem with breaking causing a rattle and squeek)