Drop-out relay

Any DPDT relay will do the trick. You run the door trigger circuit through the C and N.O. terminals of one pole of the relay and then through the output terminals of the Linear device. Run power to the relay coil through a N.O. tamper switch on the protected device and then through the other C and N.O. terminals of the relay. As long as the device is unmolested the relay will remain energized and the circuit from the door operator will be able to be completed by activating the Linear unit. If the device is tampered the relay will drop out, breaking the circuit and thereby disabling the outdoor control of the garage door.

As to sources, you can order a DPDT relay online at any of a hundred retailers, including my online store. You can also buy one at any Radio Shack store probably for about the same price. If this isn't clear, contact me and I'll draw a simple diagram for you. This one is easier to do than it is to explain.

I think what you're describing is a relay on the power that latches thru it's contacts. If it drops out, there's no power available until it is "reset". Neg on one side of the coil. Bring power in on a pole and use the "energized" contact to supply pos to the other side of the coil. You "reset" by shorting plus to the plus side of the coil. If you don't loose power, it's available for the doors. Then again, I could be totally out to lunch here but it was fun.