so if the one set of RCAs from the deck to the WS420 is bad, then the whole system's signal would be bad, right? That's where I'd start, by trying a new set of RCAs from the deck to the WS420. If that doesn't fix it, then I'd try bypassing your deck and running an ipod directly into the ws420. If the noise goes away, then you know the problem is upstream of the WS420. If the sound is still there, then I'd try bypassing the WS420, to see if that's what's making the noise.
You mentioned that you had "messed with" stuff under the dash over the winter. Is the WS420 new or was it in your system, noise free, last year?