Re: be sure to wire this device directly to the battery!
On my ride, on the driver's side I tapped off the horn relay for three relay-triggered circuits (headlight low, headlight high, ignition) each protected with a self-resetting circuit breaker. On the passenger side, other always-on circuits tap off the junction block behind the battery, as described by Bruce in the previous post.
With the factory wiring setup, the alternator provides power directly to the horn relay and battery junction terminal through 10 gauge wire. This was adequate for a 62 amp generator. I would recommend upgrading the alternator feed wire and battery-to-horn-relay wire if much more current is drawn from the added terminals.
1970 Chevelle Malibu purchased new in 1970
355 SBC, XE262H cam, Performer RPM heads, MSD HEI, TH200-4R trans, 3.55:1 Posi, digital gauges, QA1 coilovers