I'll post a detailed writeup once all my stuff comes in and I tackle this, it seems like there is a dearth of tutorials for electrical plebians like myself.
drawing's here: i.imgur.com/w6xA1gy.png
the power supply does not like to be on, but not attached to anything, so I'm going to solder in a 1/4 watt resistor across the terminals of the power supply (from +12 to ground), that way even when the relay disconnects the power supply from the bed the power supply will still have a load, and be happier than if it didn't.
There are a bunch of ways to do this and it took me a while to get a good plan. the way I'm doing it, I have a DC to DC relay between the pad and the power supply; it interrupts the flow of the power supply's current to the pad but does not turn the power supply off. It is a solid state relay, so it's more expensive, but it doesn't have any inducting parts in it, so it doesn't need a flyback diode (I was worried about the pad having something inductive in it but I was mistaken). If i had bought a mechanical solid state relay, I would have had to solder a diode in parallel with the relay pointing towards +12 to protect the inductive part of the mechanical relay from flyback.
furthermore it sounds like some people have been getting a DC to AC relay and, instead of putting it in between the bed and the power supply, putting it between the power supply and the wall. This relay would obviate the need for the resistor I'm wiring onto the power supply, because instead of turning off the load you're turning off the power supply altogether. However I'm a little skittish about cutting into the power cable, and I didn't really research this option well; it sounds easier to me to just deal with the DC current on the other side of the power supply.
I hope that helps, and if anyone notices any inaccuracies in what I said please chime in.
