Harada, that harness company, has something you can use. Some others make it as well. It's an Infra-red remote to take the place of the remote with the head unit, and here's how it works:

In the dash, the harness for the IR plugs into the factory harness that goes to the steering wheel controls. Then, a single wire with an IR at the end of it is run to the back of the car in direct line of the head unit (or inside the arm rest.... anywhere you can send an uninterruped IR remote signal). It is then programed for two things: the car you have, and the head unit you are running. Now, this only works for aftermarket head units that have remote control capability. When you press the button on the steering wheel, it works like you are using the remote control for the head unit. You just have to remember not to cover the IR transponder. A good installer can hide it pretty well. They may need to cut a slight little hole in the head liner above the back seat. In a few days I can tell you who else makes this converter. You can buy one at most reliable pro audio install shops. I think they run about 40.00