[Stang70Fastback]

Hey guys. If there are any EEs in the room, I would greatly appreciate some help. Let's get this out of the way first: Yes, I'm adding some train horns to this car. I did them on my old Outback as a fun project, and they were so hilarious that I want to add them again to this car.

Now, on to the question. What I'm trying to do THIS time around is not just throw tons of wires and relays at it until it works. I've been brainstorming ways to do the wiring the way I want, using this free online tool to draw some diagrams. I've come up with ALMOST what I want, but I have some outstanding questions. Please refer to the attached diagram.

What I am trying to do is INSERT some wiring without modifying the stock wiring at all. Basically, all I'm doing is adding a relay between the stock relay and the stock horns, so that I can use an additional switch in the dash to select between stock or train horns. The questions I have are:

1. I think what I want is an SPDT relay. My question is that this diagram tool showed me that there are Polar/Polar-Diode/Non-Polar relay types. I'm not really sure what the differences are, but I *think* the non-polar type only requires a current to flip the switch (switch flips based on which way the current flows), which is what I would want, as opposed to a polar relay, which requires a constant current to remain flipped to the alternate output, which is NOT what I want.

2. If what I've said above is correct, then I'm not sure what kind of switch I need. I think what I want is a two-position momentary switch, which would allow me to send the +12V from the cigarette lighter in either one direction or the other through that relay, but I'm not sure how that would work from a current perspective, because I need two grounds, and I'm unfamiliar with relays so I'm not sure how the wiring would work between my added switch and my added relay.

Can someone clear up that part of my design for me? Would GREATLY appreciate the help!