Basically you want a source with full charging voltage. Stay away from ignition feeds. I decided to tap into the power feed to my wiper motor since it's full charging voltage, only live when key is on, and is right in the engine compartment already. The only downside to this hook-up is that the choke is energized even when the key is on accessory since the wiper motor gets power in accessory mode. But since I rarely ever use the accessory setting it doesn't bother me any. Just one of many ways of getting the job done.