Originally Posted by ChrisSlicks

So your using the low current 12V signal to energize the relay via the switch? If that is the case then yes, the diode might help, but if the diode is just electrically noisy then it may not be a cure all. You could use unregulated battery voltage to feed the relay instead.

The white stripe on the diode indicates the cathode end (negative), I've never heard of a red stripe but they just might be trying to be helpful to mark the other end as positive. Wire the diode following the existing polarity.

Originally Posted by wstar

The diode idea might work. Depending on the nature of the little spike, a small capacitor across the switch pins might help as well (e.g. a small <1uF non-dielectric). I'd be willing to bet it's the toggle switch rather than relay that's causing the noise, but you could try doing the capacitor thing across one or both.