Help me track down bad ground

I'm pretty sure I've got a bad ground somewhere, but I have no idea where to start looking. I'm having huge voltage drops/spikes whree my speedometer will drop.fall, as will the tach, my radio will turn off, certain speakers intermitantly won't work, voltage gauge drops to around 3, etc. etc. I just put in a new batery, it guess it could be the alternator, but I think it is a bad ground. Where do I start?

Easy way to test the alternator....
Start the car, then unplug the positive terminal on the battery. If car dies, the alternator is bad.