relay control through a fading trigger?

Can the coil of a relay, for example a standard 30 amp Bosch-style automotive relay, be wired to a positive feed that "fades" to 0 volts or will the "fading" feed cause the coil to "chatter" as the voltage drops off to 0 volts? 

Yes it will chatter, but there might be an electronic substitute for the relay, contact K.P., Mr Idiot, dualsport or the Wizard, they are all good at this.

Ok, thanks.

You can use a solid state gate like a CD4093 or CD40106 that has a Schmitt configuration as the driver for the relay. The hysteresis that these components have will prevent the chatter and problems that are presented by switching relays too slowly.

The way it works is that there's a difference between the switching voltage going up vs. going down, and it minimizes the liklihood of the relay switching back and forth right around the threshold.
Just for an example, let's say it turns on only when the voltage reaches 8V. Then when it's on, it won't turn off again unless the voltage drops below 4V. So this difference in voltage, which is what is called hysteresis voltage, prevents noisy signals from causing it to toggle back and forth.

Relays actually have some hysteresis built in, but the problem is when you switch relays with such slow signals, the contacts start releasing slowly, and high contact resistance occurs. If you have a high current load, it can actually overheat the contacts as the current arcs and the contacts dissipate power. It's always best to switch the relays as fast as possible for longevity.

The solid state IC is only about 50 cents, and you'll need a diode like a 1N4001 across the relay coil to protect the IC drive output.

Now back to the WS game.:)