Now that we have discussed the various wiring harnesses out there, what is the best way to replace an old harness?
Should I just rip everything out and start from scratch like I was building a new car, or carefully detach & mark all the old connections or what? I am replacing a 79 GM harness in my 49 Ford with the EZ Color-coded harness.
Anyone ever done this?

I would gut it, start fresh. It will allow you to be neater.

First thing to do is go down the list of stuff that is electrical.Check every component,make sure it works(take note does the left headlit dimmer than the right )I mean check everything. Now its time .Remove the whole thing just cut it out and dont look back. If you try to work around someone else's mess you will have one yourself. Next see if you want any componets to be mounted diff(ford taurus has a neat swing panel for the fuse box,that you could make out of a hinge ) Think of any other component you might like (courtesy lights or? and mount them). The biggest complement i get after a complete rewire from the customer is they can see at nite with there new bright head lites. Dont splice the new harness into the old, pull the lights all apart change the plugs and make one solder joint . Also grounds ,grounds ,grounds(did i say that enoegh times?) grounds. Ground the engine to the frame ,frame to batt,body to the frame (if its a glass car lots of grounds) What ever kit you use dont put the power wire on the stater,find a better spot like a shut off switch. Good luck wiring can be very fun and rewarding when done neatly.There is a lot more to it but thats a good start
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Drive like you stole it !!

I generally start from scratch. It is less confusing having none of the old wires in the way. In most cases the aftermarket harnesses use the same colors as a GM factory harness, so having some type of GM manual is a big help. Even the all black EZ Wire harness has the GM color listed in the manual next to the wire function. I start inside the car, and wire out to the ends if using a kit, and start at the ends wiring in the the fuse panel if building a harness from scratch. Your particular car may have differences from the way the vendor intended for the harness to be used, so some of the wires may have to be moved from one bundle to another. For this reason, I lay the harness out on a piece of carpet, and plan it out before installing in the car. Make sure you don't get the wires all twisted around each other when running the different bundles, it make a tacky looking job. Last but not least, get a good set if crimpers and strippers. Mac, Snapon, Matco, Greybar, and other have good quality crimpers, buy the best you can afford. It makes a world of difference.
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Anyone can restore an old car, it takes a man to cut one up.

Thanks guys, that's what I thought I should do, but wasn't sure.
As for grounding individual components under the dash like gauges, A/C, wiper motor, etc., what is the best way?
Should I tie them all to a common ground wire, then run that wire to a good ground on the car?

I try to plan out some grounding studs and weld them in(your car is finished so i would just use a 1/4 in bolt here and there under the dash.) I also DO NOT SISTER my grounds together on the senders for the gauges.Each gauge gets its own ground wire to the 1/4 bolt. The gauge lites will be OK. But i have cured overheating cars(gauge wrong) gas gauge not reading correctly and charge metter reading wrong by seperating the grounds(they tell you in the instuctions its ok in some gauge kits but it aint) .When you do go to wire it pull the front seats out it helps your back a bunch.LO
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Drive like you stole it !!

Man! I tore out the dash & went after them old wires with my cutters!! Looked like a spaghetti factory under there.I think tearing stuff up may be more fun the building it.LOL
That new fiberglass dash is going to need a little grinding around the edges to fit in right. Of course it's supposed to be a "bolt in" deal. Yeah, it'll bolt in OK --with some modification.
Thanks for the tip on grounding lostandfound. I need all the help I can get, 'cause I'm real stupid when it comes to electrical stuff.By the way, how'd you know my back hurt?
Actually, it's my neck.

It would be "bolt-in" if you modified your car to fit the dash, instead of the other way around.

phil
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One man's treasure is another man's trash.