I have a Sirius SP5 boombox strapped to my Diad holder shelf hooked up to the battery terminals located under the dash. Some of the new Freightliner P7's have hookups for radios.
All the new package cars have power hookups under the dash, but technically you aren't supposed to use them for a personal radio. I do it anyway. The mechanics here pulled the fuses to prevent unauthorized use. I replaced it. There is a small blade-type fuse holder located on top of the battery near the positive terminal...pop the cover off, insert blade fuse, replace cover, done.
Any boom-box or portable radio that uses 8 batteries can be hot-wired into a package car. 8-1.5 volt batteries= 12 volts. All you need to do is to connect your negative wire to the last negative coil spring in the battery compartment of your radio with an alligator clip. For the positive connection, you drive a small screw into the first positive plate, and hook the positive wire to that with another alligator clip. Drill a hole into the batery cover to run the wires out of, and connect them to the positive and negative power points under the dash. I spent $30 on a cheap radio and spent about 10 minutes with some wires, electrical tape and roach clips to set it up. Its way easier than hassling with rechargeable batteries. Next time I bring my radio home I will post a picture.
If you have an older package car with no power points under the dash, look over the bulkhead door. If you have a 3 way switch up there for your light, the middle post on that switch is always hot. Clip your positive wire to that and the negative to any metal surface in the truck. Another method is to find a hot terminal in the fuse box and tap into that. Or, if you drive a hitch car, follow the wire from the trailer brake controller under the dash and onto the firewall where it connects to a circuit breaker that you can hook onto.
Whatever you do, make sure you remember to disconnect everything before you clock off!