I had the same car, a 98 S70 T5-M not too long ago so I think I can help with most of your questions.

No idea where to get the blue gauges (bad start, huh?)

The wood kit if it's aftermarket is stuck on with some foam tape stuff. Shouldn't be too horrible to take off as most of it is attached to plastic, I recommend you go buy a product like "Goo Gone" to get rid of the old adhesive. The new aluminum pieces are probably stick on as well, though I can't say I've ever seen the aluminum. I know Borton Volvo talks about it int he online parts section of their website, www.borton.com.

As far as the rear deck, I never replaced any speakers in my car, but I do remember looking with intrique at a set of 8" midbass speakers from crutchfield designed for open-air applications. Also, if you really want the car to sound great check out a post done by another owner here all about the upgraded volvo sound. http://www.jimrothe.com/volvo/volvo-dynaudio.html He also sells the kit on his website and has a copy of the original sales literature, good read in any case.

If you do decide to keep the rear deck speakers be advised that they are a very strange impedance, like 4 ohms maybe? not to mention they are wired funky too.

Don't know anything about the LED taillights, though it sounds cool :)

For Dual Rear Fog lights see here: http://www.volvospeed.com/Mods/rearfog.htm
Bunch of other cool mods over there.

The power antenna switch is pretty simple to make, I know there was a post not long ago about it somewhere, you could search that. Also, I know Borton volvo (same as above) sells the actual OEM antenna down switch kit (switch and small wiring, plugs into factory harness) for about 30 dollars. It's real simple to install.
Or, if you're interested, I actually have an extra OEM antenna down switch from my S70, however I don't have the wiring. If you wanted to make your own wiring (check the wiring diagrams, those too can be found at "www.volvospeed.com" in the maintenance section called bay 13 (GREAT spot for car main. info). I would sell the switch to you for a pretty good price, so let me know if you're interested.

The ABS light is probably THE most common problem on these cars and almost always indicates that the ABS module has failed. They can be had at the dealer for something like 750 I believe. If you want you can try and repair it yourself, instructions for that, too, have been discussed and posted somewhere on the site, or I know there are people on ebay that for a few bucks will sell you that information, don't know how happy I am about that, seems like a con.

In any case, your ABS or traction control (which I assume you don't have, as that light would be on too) won't work until you fix or replace that unit, something about bad soldering in it.

Bubbling doors is the second most common problem, or tied for first, but in any case it has no adverse effect on anything but appearance. If you're under warranty dealers replace them, free. Otherwise your dealer will fix it for you if you want, for a hefty fee of course, or they sell "repair kits", not sure how much, that consist basically of tools and glue from what I can tell, never used it though.

Finally, the door handle, that's probably just a metal rod int he door panel that came loose. The door panels are pretty easy to pop off (see www.volvospeed.com/bay13 again).

Well, good luck with everything, and welcome to the Volvo community.
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R.I.P. - Wrecked in Hit and Run --> http://www.stleasing.com/scty_boy.htm <-- '98 S70 T5-M ***** '00 C70 HPT 5-speed, NBM/Beige Full Leather, 400w Dolby PL