The diode D1 is added across the relay coil contacts right? I am assuming this, and that it is not part of the relay. Also, if I plan to disconnect the old horn out of the picture, how would I simplify the whole drawing. I will be using the power supply wire from the key on and the switched ground wire from the temp sensor. I want to eliminate the old alarm completely as it is no good. I will have to run wires from the control box(where the original alarm is located) up to the dash to make the new alarm and idiot light.
Looking at the radio shack website, they do not list a 3300uf cap that I can find. They do list all the ones in the first drawing. If I cannot get the 3300(or whatever big one I need to match my relay), would I be better off with the first diagram? Radio shack lists all the parts for that one, whether they have them at my local store is a different story.