Severe voltmeter needle oscillations

You may have inadvertently loosened or broke wire or other connections when you messed with the fuse box. I took my fuse box out to add power locks and when it was all put back together I lost the gas and temp guages. I went back and reinstalled the fuse box and everything started working again. Why you're in there look for frayed wiring which can ground out. You'll find it, hang in there.