Well, back in High School (class of 98), I had outfitted my old 85 Jetta with a Panasonic HU, 5.25" Vega Stealths (rear), 3.5" JBL GTOs (front), and a 12" Vega 124 sub in a bandpass box (argh... I know) on a Jensen 250W amp (argh... I know, again). The Jensen fried, the car is now on the way out.... Have a 97 Jetta now and was going to transfer stereo stuff.
It'll actually work out fairly well I think... Jetta has a stock HU and CD changer (which I want to keep for the time being ($$)) but the dash holes are for 3.5" or 4" speakers (stock = a foam tweeter) and the doors have 6.75" speakers (with a 2" mounting depth maximum!). I figured a few parts from crutchfield would allow me to replace the front speakers with the ones I have...... granted, power will be an issue, perhaps going from component to dual 2 ways might be a little overpowering... but right now, the highs are just a bit too bright IMHO. I have the same speakers, had to replace the amp with a US Acoustics USX2150.
My first step was transferring the subwoofer and seeing how it sounded. The HS stuff was all best buy pieces at the time. They installed it too (with the power and speaker wires together.... grrr). Now, I'm a bit more aged and adventurous, I thought about doing it myself. I even bought a dual amp kit with speaker wires (yeah, the RF 4/8 guage one) but after sitting on it for a few weeks, I realized I lack the confidence to try removing the stock HU, I don't have any familiarity with the best paths, holes in the firewall, and experience taking the car apart, and I don't know about how one installs a line out converter back there. So, I was going to have best buy pick it up, pay them the $55 + parts, and get it done.
Well, their signs say they install anything you buy, but the Tuttle Crossing one in Columbus, OH says they won't install your own wiring (warranty reasons). Well, I paid $115 for the kit from thezeb and they wanted $134 + tax + cost of RCA wires for their dual amp kit.... and I'd have to eat the cost of the one I had (I'd opened it, I might be able to sell it, but it'll still cost me a lot more). Did I feel like giving best buy 3X more money than I'd originally planned ($200+)? No, I left. I'll read up here, maybe get a guide from crutchfield, I'll consider a DIY on this (doesn't help with my original uncertainties, but confidence goes a long way).
In summary, anyone thinking about having best buy do their work, realize you'll get hosed on the wiring but don't believe those signs about installing your own equipment, it doesn't cover the wires.