Hi tableclocks,
I'm not very familiar with this topic, but I can try and help?
I believe .wmp stands for Windows Media Player? If that's the format your music files are currently in, then you could try and Google an online converter where you can upload the files for it to convert them to .mp3 (for example, I use CloudConvert at
https://cloudconvert.com/ for online conversions, but I'm not entirely sure if that website has the .wmp file extension available to convert).
Once your files are in a .mp3 format, then I think you can transfer them to your thumb drive and plug that into your truck and play them if it has a USB port?
If that doesn't work, then you could also try and transfer the .mp3 music files to a smartphone (via email, Google Drive, etc.) with a music player app installed and plug the phone into your truck with an auxiliary cord and play the music from your phone, which should then connect to your truck's speakers and allow you to hear it from there? Depending on how new your truck is, another option would be to connect the smartphone to the vehicle via Bluetooth and play it that way.
I hope one of these options helps you, but if they don't I'm sure there's another person on the internet somewhere that knows more! Good luck!