Not sure.. depends where you grounded them. Where did you ground them? Could be a weak ground. Not sure. Need more info
And yes, it is right next to the battery. Just poke around in that area under the coils. You'll find it. Just follow the negative battery cable if you must.
I just thought of something here.. maybe this has something to do with it. You said your boost gauge was kinda bouncy, right? Well, when that ground is removed, the Oil Pressure gauge [on my car] goes to full. So, maybe that ground is weak, which is what is causing the bouncing of your boost gauge and such? The Oil Pressure gage is much less sensative, and does not react to changes as quickly as a boost gauge or voltmeter would. There isn't really a need for the gauge to flop around at every tenth of a PSI of change [may seem like something is wrong with the car to a non-mechanically-inclined eye. Not good, since a lot of older folks drive a Bonneville]. So the Oil Pressure Gauge may have these fluctuations, but since it isn't quite as exact, and as apt to changes it may not show it on the dial.