PREPARING YOUR SYSTEM FOR INSTALLATION:
Most installers prepare or "prep" the system on a workbench or table before installation. The job will not only come out neater, but saves time and back pain. 20 minutes at the bench beats 45 minutes laying down under the dash!
I'd say most installers can do a bench prep in 10-15 minutes. I'd allow myself 20 for the DEI systems with the old-style separate relay pack. If you're a newbie you might easily take an hour. Just put on the radio or TV, grab a drink, lay out your paperwork and tools.
STEP 1: Step 1 is ALWAYS find and test the wires in the car, so you know where you'll be going to make your connections. Have a plan as to where you're going to mount your brain. I suggest zip ties behind the fuse box.
STEP 2: All peripherals like the remotes, antenna, shock sensor....these should go in a small box that you'll keep in the car so you have these items at hand during installation.
STEP 3: Connect all the plugs to the brain, cut off the ones you won't use (but leave a few inches of wire for future upgrades), and make your system into one cohesive thing, with all your desired wires coming out of one end.
Take whatever wires will be going to one specific location, and dress them together. For example, you know that the lock and unlock wires will be going to pretty much the same place. I like to twist them together with my drill chuck; others prefer wrapping them with black tape.
Anyway........
The goal here is to do as much as you possibly can at the bench, and and leave as little as possible inside the car.
Your prep should allow you to do the following:
Jump into car, loosely put brain where it's going to go. Maybe just jam it in there, maybe hang it loosely with one zip tie. Don't fully tie it in yet; you may need to drop it down again to correct potential errors.
You should be able to immediately grab a bundle of wires you've already pre-dressed to be together, run them to their appropriate location, RE-TEST the wires you found, connect the wire, and tape it.
Really that should be it, just run wire, retest, connect, insulate, and on to the next.
ABOUT THAT SATELLITE RELAY PACK: I hate those things. Adds installation time, and makes it hard to keep the system neat.
You'll have a decision to make here: You can choose to Velcro or double-stick tape or zip-tie the relay harness to the main brain, making it into one single system to install. Or, tape any necessary other wires along the ribbon cable, and then you'll have two separate things to mount.
I suggest doing a google image search, and/or look closely at any used DEI remote starters for sale on eBay. The used systems will already have been "prepped" before, and you can see how another installer did it.
Just some examples......
http://www.titantalk.com/forums/titan-parts-accessories/89405-viper-5701-help-remote-start-security.html
In this picture, you can see that someone just jumped into the car and started hooking up wires. Not only does this job look horrendous, but the person is probably spending hours and hours crouched under the dash unnecessarily.
http://forums.s2kca.com/showthread.php?31150-FS-Python-Remote-start!-Installed-your-garage!
This system has a little more tape than I would use, and the installer chose to have two bundles of wire coming out instead of just one, but you'll see how the wires have been routed so they pretty much come out on one end.
http://forum.sounddomain.com/ubbthreads.php?ubb=showflat&Number=487929&page=99&fpart=2
This prep is from a very experienced installer; I know him. Note the wires spun in the drill.
http://www.d-series.org/forums/audio-electronics/96234-keyless-entry-installation-basics-lots-pics.html
Many installers like to screw the brain to their workbench. You could also put it in a bench vise. Either way, this site offers a full pictorial of bench prep.