We're not getting much in the way of details about the truck. I get the impression it is a rod/custom vehicle if it has an AM/FM radio and is posted in the 1937-1945 forum. Unless it was a very unusual or custom unit the only radios available in that time period were AM. I am assuming that because is has an AM/FM radio it has been converted to 12 volts.

If there is an FM radio then you can easily add a Sirius receiver that outputs to a small FM transmitter. You tune the radio to the frequency of that transmitter and it broadcasts the Sirius program to the radio.

I have done that for many years in multiple vehicles that did not have a radio with an auxiliary input or a satellite ready receiver. There are 2 limitations. One is that the quality of the signal is not quite as good as a direct input. In a vehicle going down the road that is not noticeable. The real pain is that as you travel to different areas there will be FM stations broadcasting a strong enough signal that it will override the output of the Sirius FM transmitter. So you will need to change the output frequency of the Sirius transmitter and retune the radio to that frequency.

Another way to get Sirius in a vehicle is to get a Bluetooth receiver that transmits an FM signal. Then you can connect your phone via Bluetooth and listen through the Sirius app over the internet. The limitation here is that this requires a decent cellular signal. It is not coming directly from the satellites. The real advantage is that you do not need to buy a Sirius receiver and get a new subscription. Most subscriptions today include internet access. Plus you can listen to playlists on your phone or from other services like Spotify through that Bluetooth unit. You will have the same "retuning" issue as with the FM transmitter on the Sirius receiver.

If the radio has a cassette player you can get an adapter that takes the output from the Sirius receiver and sends it to the head unit of the cassette player. It works but the sound quality difference is noticeable.

I will offer one piece of advice that many of us share with new vehicles owners. Unless you know the vehicle very well, be ready to spend a lot of time on the basics. If it has been sitting a while I suggest that your greater focus needs to be on all the basic operating systems and mechanicals of the truck. For example how old are the tires? How well do the brakes work? Is the steering nice and tight? What is the condition of the electrical system? The last item I would worry about is the radio.