Bob, you are mostly right about the need for the diodes. The only time when they are necessary is when the back porch light is on and the truck is in reverse. As a practical matter that would almost never happen. I would have to leave the back porch light on and then be driving the truck. On second thought, I am a forgetful sort, so it could easily happen.
Without the diodes the truck starter battery would be turning the light on through the transmission switch, and the house batteries would be turning the light on via my 2 way switch as well. The net effect is that the 12V positive of the starter battery is connected to the 12V positive of the house batteries. That would be no problem if they were the same exact voltage, but they would most likely not be at the same level of charge, and if the truck were running the alternator would be charging the starter battery as well. The result could easily be a 2V difference which would drive a current limited only by the wiring resistance. The resulting current could easily be 20A or more, which would immediately blow the fuse on the trucks backup light circuit.
As for switches, there are many different kinds, and I find the terminology is often confusing. Fortunately, in this case, I am talking about one of the simplest kinds of switches. (I won’t even try to tackle some of the more unusual ones). I double checked the terminology for Single Pole Double Throw (SPDT) using Wikipidia as follows:
http://en.wikipedia.org/wiki/S...Contact_arrangements
As you can see it goes by many names. The British call it a 2-way, the Americans call it a 3-way, and it is also sometimes called a single pole changeover switch. The simplest form does not have a stable center position where neither circuit is connected, but most types do have what is called “break before make” which means that both circuits will never be connected at the same time. Some switches do have a stable center off position, and they are sometimes called Single Pole Triple Throw though I would argue that a SPTT should have 3 possible circuits, not 2. The term SPCO (Single Pole Change Over) is sometimes used to denote a center off position as well. Bottom line, when I say SPDT I am talking about a switch with only two stable positions.
I think I confused you when I referred to a SPDT switch as having 3 terminals. I was counting the “common” as one of the terminals (I was literally counting the screw lugs).
Clearly words are not the best tool to describe these things, which is why I provided a schematic. It can be found in the photo gallery under keyword “Stealth” and is, for the moment, the most recent photo posted.
By the way, your explanation of how you wire a two switch light circuit is right on. Thanks for clarifying.