I have limited experience with relays, diodes, ect. so I need a little help. I'm currently building a rock crawler (off-road buggy) from ground up and I'm into the electrical and need some suggestions on turning on my gauge lights(4). I would like to have the gauge lights turn on only when I have the headlights and/or my rock lights (undercarriage lights) are on. Both sets are operated by separate fused SPST toggle and relay. I realize that if I tie into both relays then power would back-feed to the other circuit. I think a couple of diodes would be the fix but have no clue where to start with those, so I dunno. I know the easiest way would be to just put it on a toggle by itself or just tie into the ignition circuit, but I like to complicate things. Hopefully someone can give me an nice simple solution. Thanks!
It sounds as if two diodes will do the trick. You will want to purchase diodes that are large enough - 1N5402 diodes are good for 3 Amps and should work nicely for gauge lamps.
Each diode has a band at one end. Connect the ends of the diodes with the bands together and to your gauge lights. The free end of each diode goes to one of the lighting circuits that you want to take power from.
Do note that this will introduce a slight voltage drop to the gauge lamps (about 0.8V @ 1 Amp) but I don't think that you will notice the drop in brightness.
If it turns out that you can't afford any voltage drop at all, come back and modify your original question to say that. You can do the same thing using a SPDT relay with 12V coil but it costs more than these two diodes.