Jazzmaster Grounding Issues

I have a Squier J Mascis Jazzmaster, and I was a little to eager to switch the gold pickguard to a mint one. I thought that this would be a cosmetic change without any problems, but when I set it all up again, there were some things wrong. The pickup selector no longer works in the rhythm circuit, and the rhythm circuit is much louder than the lead circuit. Also, the neck pickup is louder than the bridge pickup.

I have learned that this is probably due to issues with grounding. The problem is that I have no idea how to fix this. Can anyone help me out with what exactly I need to do to ground the guitar?