The terminology used is rather confusing.
However I assume that you require to draw the maximum load possible, while ensuring that any connected load will be disconnected when the ignition switch is turned off.
Your best and easiest approach will be to establish a completely new circuit directly from the battery by means of a heavy wire, importantly with an in line fuse fitted as close as possible to the battery terminal. Do not leave out this protection.
Incorporate a heavy duty 12 volt relay with a normally open contact, within your new distribution fuse board, to switch the new separate supply. Then control this relay via a feed from fuse No. 3. The cigarette lighter circuit is usually used because of easy access.
An existing circuit used for audio is another alternative. Any under dash stuff will be fused and can be used. Simply test for voltage with the ignition switch in the accessory position, as kwren has suggested.
The above arrangement will be electrically perfect and will involve the minimum of damage to existing loom wiring. There will be the very minimum of voltage drop, which appears to be a major concern due to an expected a high current. Furthermore, it will prevent an extra heavy load being applied to the ignition switch and circuit.