Big coincidence!...I think
I don't think the snow underneath your car and the wires inside your car have anything to do with each other. Unless you ran your wires next to your drive shaft. LOL
You'll have to narrow down the problem yourself by swapping in working components. Eliminate your back speakers. Disconnect them and run a pair of properly working ones. If you still hear the noise then grab another amp and hook it up in place of yours. If you still hear the noise then hook up your amp and speakers in question, find a car with a system and park trunk to trunk.
Unhook other car's RCA cable from amp and run it to your amp with speaker. Play music from the other car with your amp powered on. RCA from other car + your amp and speaker and no noise...then you know the problem must be your deck or amp wiring. Amp could be wired right, but with low quality wires, could have broken in the plastic sleeve or at the tips, could be pinched. Also make sure you didn't run the RCA's next to the power wire. That's all that I could think of with out actually working on the car myself. Let me know how this turns out.