Depends.
Like my Pioneer radio have a funky 6 prong port on the back that you can get an "iPod" adapter, officially from Pioneer for it. Unfortunately, the male end that plus into the device is iPod specific.
I know some head units literally have a USB 2.0 port on them. If this is the case with yours, you might be able to use the cable that your phone came with. It's called a USB micro cable.
If your head unit is like mine with a funky adapter though, I suggest seeing if anyone makes an adapter that has a male standard head phone jack.
That is actually how I had to hook mine up. I had a spare set of RCA input's on the back, I just hit Radio Shack and bought a 6 foot wire/plug that was a red and white pair of RCA jacks on one end and a male head phone jack on the other. Plugged those into my RCA inputs and I plug the headphone end into my EVO and poof!
Post the brand and model number of your head unit if you need more help.
