I have a situation where I need to feed power to an automotive radiator fan from two power sources. First off this is signal power only, not motor power. So we are talking less than 0.5 Amp here. The first source is for power to be fed with the Key on. For Mercedes Benz vehicles this would be called circuit 15 power. In a second case the power would be fed with the key off directly from the car battery through a thermal switch that would allow power to flow whenever the radiator temperature exceeds 110°C. For Mercedes Benz this is called circuit 30 power. The problem is I don't want circuit 30 power to activate circuit 15 when the thermal switch activates. Since these two circuits are not carrying fan power I was thinking a diode or similar device put in line with circuit 15 might do the job. I could do this with a relay that switches off circuit 30 and switches on circuit 15 whenever the key is on except if the temperature was above 110°, which could happen while driving then circuit 30 would be connected to circuit 15 which seams like it would be OK until some high power device on circuit 15 decided to feed from the tiny wires going to the radiator fan controller. So my question is what is the best way to accomplish this and if it's a diode what size and type.