Don't forget, wherever you squirt WD40 in you'll get rainwater in there anyway, and WD40 is a lot less hazardous to electrics than rainwater is!! Generally WD40 is pretty safe anywhere on a car except for your brake discs and glass, for obvious reasons.
Personally I'd have a look at where it's rubbing, and use a bit of lithium grease. It's a little less messy than WD40, a bit more permanent, and may stay in place a bit longer. Mine squeak towards the end of the fold, I can see it's where the wing mirror casing isn't quite fitted right and it rubs against the base.