There are a couple of options available, 1 is if you have a tape deck, to use a tape converter, it has a headphone wire coming out of it, and will plug directly into any headphone jack, and the sound quality is very good. You can use an FM Transmitter, but frankly they suck, go in and out, only get FM quality sound, and you will get a lot of interference with it. Another option which would work really good, would be to use a device that hooks up directly to the radio's antenna, in between the antenna and the radio. They make them with headphone wires that come out of them, and works on the same idea as the FM Transmitter, but will have no interference issues, and good sound quality. This method is generally the best, but in a truck you will most likely have to pull the radio out to put the box in.
When I was with swift, I took the factory radio out, and put my own in, my radio had built in support for mp3 devices, an input jack, etc. Plus I had 1000 watt woofer under my bunk
. All of which only took me maybe an hour to install, and 10 minutes to uninstall and re-install the factory radio. No way to tell the difference
.