Oh thanks! I did some tests after your recommendation, and you were right! For the cable, you don't need stereo, mono works fine, just using a simple adapter for 1/4 to 1/8 works! But yeah, the way the spider 3 inputs are wired, it appears that the signal of the mp3/cd input is somehow connected with the guitar input, so if there is no guitar cable plugged in guitar in, the circuit is broken, and no sound! But just plugging in a dummy 1/4 to 1/8 adapter in the guitar jacks fixes things.
So just using a 1/4 cable with a 1/4 -> 1/8 adapter to cd/mp3, BUT it requires either a guitar cable connected in amp guitar in of the amp, or in my case, just a 1/8 -> 1/4 adapter plugged in guitar in.
My goal was to eventually use normal speakers, as the L6 Spider isn't the greatest, but yeah I think this will work much better than PodGo to amp's guitar in! Thanks! :D