You don't want a constant 12V on the coil because if the transistor that switches it fails then you will approx 10ohms or less across your battery.
2 things will happen.
1- your battery will discharge quite rapidly.
2- the wiring may heat up and lose its insulation if the coil resistance is less than 4ohms

12V\4 = 3A

If you can't find anything already there then your best bet is to add a separate wire grunty enough to cope with about 8-10A (to be safe) that is fed from the ignition switch.