Finding and connecting constant 12v

I am going to be installing a CD changer in my 2000 Volvo S40.  I have purchased the peripheral adapter that allows me to run the changer with the factory stereo.  However, the adapter has a yellow wire that needs to be attached to a constant 12volt power source.  I ahve never installed a stereo before so I am unsure how to to do the connection.  I don't have the wiring diagram, but I should be able to find the correct wire going to the stereo using a multimeter.  The problem is once I find it, do I just splice into that wire and connect it to the changer wire, or do I get the constant 12v source from somewhere else?   Thanks

You can splice the wire and join it, or you can get a wire-tap connector from a local auto store (or even home depot) or your third option is to connect a wire directly to the battery for the constant 12v. However make sure you put a fuse somewhere between your changer and the battery connection to abide by the safety precautions. Whether the changer kit might already have that fuse or not, I would suggest putting a fuse anyway.

wire taps are garbage. Solder to a pre-existing wire and fuse it.

Where would I put the fuse?  The wire from the adapter is only about 4 inches long.

I don;t know if wire taps are garbage or not, even though I personally decided to agree to that. Just as info, my Toyota dealer charged me 450$ to install a Automate alarm in my previous car, the charge being for the alarm and installation charges. What I noticed is that the "expertly trained" dealer put wiretaps. Go figure. It really made me think if these are good or not since a soldered joint is the best.

About the fuse location, since your wire is only 4 inches long, you will have to join another wire to it unless you can get an inline fuse with 2 wires that you can connect to the 12V and to your changer. When a wire is too short, you just have to join another to make it longer.

By me saying wire taps are garbage I should have stated that is a personal opinion. I started working at a ship where the last hack er installer used t-taps and even worse just strip, wrap and tape job. I even ran accross a couple vehicles where there was no tape just strip chaulk covering the connections. That kinda experience can realy make you resent something. I also dont like wire taps because they generallyy acctually cut the wires making them weaker and comprimises their integraty. Anyways back to the above question, just like what markcars said, just extend the wire and fuse close to the wire you are tapping into. 

take the constant power from the deck for that

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---- 1996 Chevy S10 ----
1 Lightning Audio S2.600.2 Amp
2 Lightning Audio 12" Subs
1 Lightning Audio 1 Farad Cap