I'm adding a 6-ciruit fuse panel to my '70. The plan is to run a small gauge wire from my existing fuse box (key on power) to a relay and run a heavy wire to the relay and then to the input of the fuse panel. I plan to mount the panel under the hood, so the input wire will not be over a foot or two long. The circuits are: taurus cooling fan, power seats, power door locks, sub amp (150 watt), and maybe power windows and some small accessories in the future.

Will the radiator fan controller need key-on power or constant?
What size wire do I need as input to the panel?
Should I tap power at the horn relay or directly at the battery + terminal?

Thanks for your advice!

Sounds like you are on the right track by only using the current fuse panel to switch on the secondary panel.

I would recommend connecting the secondary panel supply to the horn relay as that is the main electrical bus in these cars. Normal current flow will be from alternator to horn relay/bus and then out to all power consumers.

The radiator control can be done either way and there are pros and cons to both. I wired mine up as always hot with the thinking that it could turn on and cool the engine and engine compartment after shutdown. Some think this is a waste since the water pump is not circulating coolant and could just run the battery down. I think there is enough thermo syphoning of coolant to provide some cooling and observe that the fans can run for about 5 minutes before shutting themselves off.

Regarding wire sizes, I do not have any of my references handy but a web search for "wire current capacity" can turn up a number of resources. Just remember to use circuit wires that correlate to each circuit fuse and a supply wire that correlates to the sum of the circuits or to some other shared common fuse.