There are many threads on internet fora (forums) about performing electrical modifications to cars. If you are serious about what you are doing and you want the installation to be reliable you need to have the right tools, the right parts and the right attitude.

I can't provide everything in a single post so this is by way of an introduction.

ScotchLocs:
In my opinion these are totally unsuitable for a high quality permanent installation. They are a type of connector called an Insulation displacing (IDC) terminal. They work best on solid core wires, as found in telephone and Cat5 Ethernet wiring. The vast majority of car wiring uses multi strand wiring and IDC terminals simply move the strands out of the way resulting in a relatively poor contact. Use too small a scotchloc and you can cut the strands of the wire causing a permanent hazard, too large an item will make for a poor connection or even no connection.

Insulated terminals:
These are an excellent means of making a secure connection, providing the correct tool is used to crimp them. Crimped terminations are reckoned to be better than solder in most cases. Insulated terminals should be used in conjunction with heat shrink sleeving where a water resistant connection is required.

Un-insulated terminals:
Again an excellent means of making a connection and necessary for adding items to fuse boxes, obviously the correct terminals and tools are required. Un-insulated terminals require a different crimper from insulated ones. Again heat shrink sleeving can be used to reinforce the joint if required.

Tools:
Wire strippers:
If you want to make a high quality connection a decent set of wire strippers is essential, the best types have specific dies for each size of wire. Many cheaper tools tear the insulation instead of cutting it or will damage the wire strands. Once stripped the wire strands need only be twisted slightly to restore the natural twist imposed when the wire was manufactured. Twisting the strands tightly together will make for a less secure connection because it doesn't allow the terminals to be crimped correctly.

Wire cutters:
Side cutters are the most reliable but buy the best you can afford, cheaper ones don't cut the insulation cleanly.

Hot air guns:
If you plan to use heat shrink sleeving (probably called heat shrink tube in the USA) use a hot air gun to shrink it into place, a soldering iron isn't really suitable. Hot air guns are cheap and effective.

Crimping pliers:
You should use a ratchet type plier to ensure a consistent, high reliability, termination. Crimping pliers without a ratchet tend not to be closed tightly enough and result in a poor, easily disturbed connection.

Please feel free to ask questions.