There is no way in hell you would catch me putting 15V into my 12V power supply! It would probably be better wiring directly to the battery.
I'm guessing you have a Morex DC PSU (long and slim), with the ILAN AC brick?
I have that setup, and I have the PSU wired directly into the battery, and it has been fine for the past couple of months. No guarantees it will also be fine in your car, and no guarantees that it will work at all, try it at your own risk.