In the IT world, as well as in the Audio world, there's a simply rule of thumb.
"95% of the time, it's the cable!"
In this case, the cable works on your laptop, but not in your truck.
Here's the steps I would take to diagnose:
- Try a different cable. Result?
- Using another device (mp3 player or laptop) with a 3.5mm port, plug that cable into the new device and into your truck. Result?
- With that cable plugged into your truck (only), turn the volume up and touch the open end of the other side of that cable. You should hear a varying HUM in your truck's speakers.
- Keep in mind that not all 3.5mm connectors are created equally. Some plugs and jacks just don't "mate" well. Sometimes there's some plastic that gets in the way of a plug going in all of the way. This is why even a good cable may not work in certain instances.
HTH