I bought a 10 pack of them a while back on ebay.
http://www.ebay.com/itm/10-PACK-RELAY-HA...cc8&vxp=mtr

I ran new wire to the vr, and already had new wiring to the alt.

in your case, I would trace back the line to the ballast, clip it there. see the drawing above.
that way both the vr and the other field line get 12 volts.

I wired the switch portion to that line, so when it got power the relay closed.
power to the vr and alt came from the starter solinoid as it was pretty close to where I put the relay.
I grounded it to the vr/engine ground strap.