Hi all I need to find ignition wire as I need to connect t to my ower supply. This should be behind the key lock right? How do I approach to it and how to know which one is right wire? Truck s Mercedes Atego 2005 year
I wouldn't necessarily jump to the stereo wire; this would depend on where your unit will be mounted and what it is. As stated; you shouldn't draw excessive amounts of power (>2-3 Amps or so) and it should be fused. If you will be needing lots of power then use the ignition wire to switch a relay with battery power, not to provide power itself.
If you're mounting it behind the stereo; then by all means use the stereo. Yellow is usually constant 12V from the battery (unless Mercedes is different in this regard) and Red is usually switched (only provides 12V when the ignition is in Accessory or On).
There may be other reasons you would not want to use either of these; first if the unit will not be anywhere near the stereo and second if you need ignition-on only (off for accessory).
My first stop would be the fusebox. That will contain most sources of power to tap into; accessory and ignition-on, ignition-on, and start. You'll need to find an appropriate fuse (avoid important ones like 'fuel inj.', etc.) and tap into the fused side if you want to do this; they even make terminals for such a job that straddle the fuse pin and present a terminal above the fuse. If you get lucky, there are many times empty slots with a live pin on one side that is the type you are looking for, so you can add your own fuse and not piggyback on another circuit's.
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What are you trying to power? The ignition wires are generally only intended to power the ignition - if you want to run anything else you are better off taking an appropriate feed from the fuse box, or a relayed feed from the battery, depending on the power draw of the intended application.
Either way, the best place to look is a wiring diagram for the truck in question - usually found in the back of any decent workshop manual. This will give you the colour codes of the wires you are looking for - ignition-fed live feeds are usually red, but this does vary by manufacturer.