Under the dashboard of the car is typically a fuse box or fuse panel. This is where to tap the wires. Start with the owner's manual. This should list the fuses. As already suggested, one good generic place to tap is the radio circuit. You can trace the power wire from the back of the radio and tap it there. If your current draw is small as in less than 1A, everything should be fine. (The worst that can happen is that you blow the fuse for the radio and have to replace it. )
If you do not want to use the radio circuit, look for a feature that is not implemented on your car. Something like, power antenna. Because the wiring harness must support all the features, it will have an unused connection in the fuse box.