This topic has been brought up sooooo many times. And the conclusion has always been- "it worked for me"...but is it the best way?
I was actually in the 12volt industry for a number of years- started out installing stereos to put myself through college...and ended up working for a couple of the bigger Car Audio manufacturers. So I do have some thoughts on what I was taught by the industry.
The engineers of this car- my example pics- weren't influenced by the been counters as some of less expensive cars. So- I'm thinking this was done right. Please let me know if you disagree...and why.
Here's how they - or rather a team of engineers from Germany- wired this car w/ not only powered and heated front seats - but the rear seats as well.
Three large gauge wires can be found running down the side of the car- the battery is in the trunk. The largest gauge wire from the battery to the starter- the next smaller - runs to the alternator 140amp. (Sidenote-The Japaneses version has TWIN alternators- since it sits in traffic a lot- the second smaller alternator -is driven via a separate pulley /belt so it turns full speed at idle when the AC is turned on-sorta confirmation they -engineers- thought of everything) The smallest wire runs to the fusebox under the hood.
Another interesting electrical feature is the positive junction under the hood- where as the owners manual states to ONLY jump the car from that point- not at the battery were it may damage the electronics.
Here's some pics- this is a BMW 750 IL V12- new (1997) MSRP was $98K.
So...where am I going?
I do NOT recommend using the lug on the alternator as anywhere near the best source to pull power. Remember these cars were built way before 500watt car stereo amps. electric cooling fans and heated seats were even thought about. It is not good for the alternator to deal w/ surges-such as caused by electric fans turning on. Headlight filaments don't last as long. And electronics hate surges.
Notice the large junction block (White plastic) next to the battery-that's what I recommend-get your power right off the battery.
The battery does more than just start your car. I helps w/ surges-stabilizes the voltage in the electrical system and filters the AC ripple not cleaned up by the alternator. Electronics like clean power- ever heard of "alternator whine?"
Here's some of the junction blocks I like-
This came off a BMW 7 series-twin relays-5 fuse- can use the relay to power the junction block only when the ignition is on. Less than $10 at you local junkyard
This is my favorite- stackable- can custom wire the relay- and set up w/ 3 fuses- which you can also custom fuse- relay output or the source power.
And I found a nice sealed plastic box- out of a 7 series BMW that will work great for relaying the cooling fans/electric headlight motors and headlights.
And if looking at headlight upgrades- get some good connectors-these are ceramic and supposedly won't melt.