Look at the rating on the AC adapter for your iRiver, then just buy a cigarette lighter adapter with similar specifications. There are universal ones (heck, even Radio Shack probably has them) that will let you set the correct voltage. Then you can wire it straight to the battery if you don't want to mess with the cigarette lighter port.
I have a Pioneer head unit with rear auxiliary inputs, and rear is definitely the way to go. For one, it's usually RCA-type left and right inputs, where the front input is usually stereo 1/8" inch jack. Also ease of removing the faceplate, as mentioned.
I have my iPod hooked up to a Belkin car charger (which also provides line-out, a feature of the iPod dock connector). So I just hop in the truck, drop the iPod in its cradle, and I have tunes and charging with no plugging/unplugging or wire routing. Sweet