There are typically 3 places where you can obtain power for your changer: main ignition harness, fuse panel or rear of radio. CD changers do not require much current (about 2 amps) so the radio is where I connect them. Your owners manual should indicate the max power consumption. If the current draw is more than 5 amps then I would use one of the other 2 chioces.
If you are mounting the changer in the trunk, say under the rear deck, the "interface" wire should be routed from there, down the rear passenger side shock tower, under the rear seat, along side of the factory wiring* under the carpet, and then upwards to the hush panel under the glove box. This is a good spot for the modualtor. You can make all of your connections, cable tie all wires, and it'* all hidden by the hush panel.
I hope this helps.
* Do not run audio cables too close to any higher current wires. These will be a heavier gauge. You can run into interference in the audio is you do.