How's it going?
You are right. Abolutely replace the line level input "speaker wiring" with shielded cable. You see, with your current amlifier audio input wiring configuration, those unshielded input wires are kind of like antennas, picking up any stray electromagnetic noise (EMI) and radio frequency noise (RFI).. The mobile environment is full of rfi and emi, and all of that noise is feeding right into your amps...
Devices that produce low-level audio output, such as tape decks, mp3 players, carpc sound cards...must have their low-level outputs connected using shielded cables to the audio amplifiers that drive speakers in the car or wherever.
Here is an example from my own carpc experience and installation:
My carpc is located in the large rear cargo compartment of my Saturn wagon (see my pictures). I use a Creative Sound Blaster Extreme PCI card, with fron, rear, and sub outputs. I use Monster audio cables to interconnect my sound card to my audio amps inputs; 1/8" mini stereo plugs on the sound card end, stereo RCA plugs on the audio amps end.
I had some noise on only one out of my three audio amps that I was unable to correct with changes in grounding, cable, re-routing and such. So, I bought and inserted a Radio Shack in-line ground loop isolator (GLI), cost $16, and the slight noise not only went away, I had NO reduction in overall sound quality.
I tend to buy a great deal of my amps, cables, etc, etc for audio hobbying from
Parts Express http://partsexpress.com
Good prices, great tech support, what I need...
Best of luck and blessings. Keep learning, keep asking...