I want to add a switched circuit for accessories using a relay instead of wiring them directly to the ignition. Basically, the coil of the relay will be wired to the ignition (i think it's the blue wire but don't quote me), and switch battery current (via 6 or 8 gauge wire) to a small fuse panel. Is this the "right" way to do this? I always thought accessories in cars (such as the radio) ran off the main relay, but that isn't the case in the Trooper. I don't want to add a bunch of circuits to the ignition wiring without a relay. Right now, it will only be used for a heated o2 sensor and my 400w inverter (this also works through the cig lighter but gets hot above 150 watts load or so, hence the heavy wiring).

What's the best way to tap into the ignition wiring?

Are those insulation-displacement "vampire" taps (Scotchlocks) advised against?