Wiring help

As the question will prove, I'm not an electronics genius. I installed a headlight relay harness a couple years back to brighten up the stock headlight a bit among other things. When I did that, I ran an extra fused line directly from the battery to the front of the bike so I could put on grip heaters some day. That day is rapidly approaching. But I want the heaters off when the bike is off.

So, can I tap into a switched line on the bike and put the trigger side of a power relay in series on that line w/o screwing up anything else? From there, I could run from the relay to a hi/low/off switch for the grips. switch would always get power when the bike is running, but I'd still be able to control the grips themselves with the switch itself.

If this would work, anyone recommend a good, non-critical switched line I could tap into either under the tank or up in the headstock area? I say non-critical, b/c if the relay trigger goes bad, I would lose that entire circuit, right? Maybe the line from the running light that is always on when the bike is on? bike is an 02 M620.

Thanks,
Mike