That's great! I thought all the fuse are constant.
If there're some switched fuse, the add-a-circuit is the best tool!
Burried in this thread: I need a wiring diagram please...
There is a link to the full schematics of a 2008 Malibu here:
Pg 62 of that PDF has the switched fuses that are in the right side center console BCM.
Depending on the model you have, I would suggest using the fuse location for the Heated Seat/Roof fuse, since, it is possible you don't have heated seats or a power sunroof, and even if you do, you should be able to tap this fuse with an add a circuit, put a 10A fuse in for your power tap, and a 10A fuse for the roof/seats. This would put a 20A max load on the main circuit, which is probably OK. You Might want to be careful if you actually have heated seats, but this circuit appears to only be the power for the control portion of the roof or seats, not the actual current for the heater or roof motor; it also seems to protect the aux power outlet switch.
In general, be conscious of how much extra load you put on a tapped circuit, as the underlying wiring on the battery side of the fuse may not be properly sized for the extra current draw you add and you could create a fire hazard. Really, according to this simple table
the underlying circuit should be 14guage wire if carrying 20A. Not sure what is used in our cars for power distribution, but you are likely safe tapping at the fuse panel with an add a circuit and 2 10A fuses in it. Then, use up to 15 feet of 18 gauge wire to run from there for power to your circuit.
Disclaimer: I only interpreted the tables and the schematics, I did not do an actual engineering assessment of your circuit plans, load, and whatever, so use the info above at your own risk.