I recently bought a 14 year old SUV. A Mitsubishi Montero. Most of the electrical on this old thing had been damaged and or hacked into long before I got to it. Little by little I've gotten almost all the electrical back on line... Recently I decided to put a lighter in the dash. The lines going to the original one needed to be re-ran and I was not about to take the dash apart again, so I snaked a new line through to the fuse block and piggy-backed a line off it; I needed an in-line fuse but it was 2:00AM and no auto-parts store is open then so I made my own. It is very simple, but it works well and it cost me nothing because I already had leftovers from past work.
Step 1: Parts...
an automotive fuse (whatever the correct amp is for your project). a box of em' is only a few dollars.
some wire (heavy enough gauge for whatever you are hooking up). I had 7 feet that cost around 27 cents a foot.
solder and soldering iron. a decent soldering iron is not too expensive... at most around $20
heat-shrink and a lighter. heat-shrink was about five bucks for a big coil of it. ..... lighter, I found it in a parking lot.
zip-ties (if you want to tie the new wire up alongside wires that already are under your dashboard.) a lil' bag is about $7 and lasts a while.
Step 2: Connecting the Tabs of the Fuse to Wire.
So I decided to first solder these pigtails on and then afterwords solder these pigtails to the main wire so that I was able to first run the main wire and then simply splice into it in a spot that would be easy to get to if the fuse blows.
Ohhhhh, and by the way, I am not to blame if you jack up or in any way damage your car and/or electrical system. If you do not feel comfortable messing with the electrical system of your car, then don't do this.