Splicing wire is not an art, it's just plain ol' logic. So think about what you're doing, and why. This has to last the life of the vehicle in some pretty harsh conditions and NOT let you down. If you've got the time and money to do it twice, or three times... then you've got the time and money to do it right the first time and forget it.
<font color=red>ANY</font color=red> connection you make that's not soldered, sealed and heat shrunk can become compromised by corrosion. That's a problem. Don't use no scotch-locks, no fancy connectors, there's no easy way out! Use GOOD quality multi strand wire, and get a gauge that will handle the current return you need. Make sure you use good quality ELECTRIC solder, it's the resin core stuff. If you use the mechanical acid core you've just destroyed the harness.
Buy several sections of shrink-wrap tubing (available at Radio Shacks and most hardware stores). Cut them to the length you need and slip it over one of the wires you've cut and will splice.
When you make a splice, solder the wired together. It's good electrical practice and keeps the joint from becoming intermittent.
Now fill the area of the splice with RTV or something like Permatex black, and slide the shrink-wrap tubing over the area of the splice.
Heat the tubing, and it will shrink down, extruding the sealant and forming a very nice, professional, and weatherproof splice that you'll likely never have to visit again. That's what you want, do it right the first time and forget it!
Now visit the electrical panel in your house. Look at all the wires that are slipped into holes in the big Bars. One is a grounding bar and the others carry the voltage. This bar is a buss Bar. It's just a bar with holes and bolts that thread down into the holes. You slip in a bare wire, tighten the nut down on the wire, makes a very good connection.
If you put one buss bar on the firewall, and a second one on/under the dash you now have a CENTERAL grounding buss bar for both locations. This is the place where ALL grounds should come terminate. The is type of bar can even be enclosed to help eliminate corrosive factors such as road salts and chemicals. That's your choice, but it would be a good thing to do.
Here's one of my favorite pictures of all time. If you've never seen <font color=blue>slnewcomb's</font color=blue> answer to a grounding problem, then you're in for a treat. Take a look at his grounding bar, this is buss bar has been done right:
Some think it's overkill, but IMHO it's better that clean underwear and sliced bread and prettier than a naked lady (unless she's holding a gadget).
It's a real piece of beauty, a piece of the mechanical arts! <font color=red>It's done right the first time.</font color=red>
Fratt77CJ, this may help you out: <font color=red>Painless Wiring Instructions</font color=red>