I just did this in a 300TDt a couple weekends ago so the info is pretty fresh in my head. Those 4 speakers wired in parallel with a common ground will provide a 2ohm load to your new head's amp and it probably won't put up with that for very long, plus modern heads have fader controls onboard so I'd recommend definitely taking the factory fader out of the equation and getting all of the wires up to the radio where you wil forever have all the options you need. I didn't run new wires to the speakers - the factory wires were a heavy-enough gauge and all in good condition.

Here's the basics of the factory wires:

Brown = Ground -
Black = Switched Ignition +
Red = Always on + (memory)
Grey = Dash Lights (dimmer)
Blue = power antenna

The speaker wires you'll find behind the radio are;

Red+ Black- Left Speaker (goes to fader on console)
Green+ Black- Right Speaker (goes to fader on console)

To bypass the fader:
. remove the ashtray
. remove the two screws from the ashtray mount and pull that out
. the center console slides back an inch or two and can then be lifted. One mine, the front corners had to be bent slightly (be careful not to break anything) to come out from under the plastic bits and be careful with the back (the end closer to the armrest) - there's a pin at the very rear that goes into a hole in the console mount - if this pin isn't fully free of the hole and you yank up, you'll break something

There is a plug on the bottom of the fader unit that has 6 wires going to it - 4 speaker + and two common grounds. Pull the connector off the switch and snip the wires off the connector and then pull that entire sheathed bundle back up under the console into the hole behind the radio spot. I had to remove a large zip tie near the shifter and there was masking tape around a few of the wire bundles way underneath that I basically had to tug that bundle out of.

Once you have the whole bundle available, carefully remove the sheath from the wire bundle and what you need to do from there is pretty obvious. Clip the connectors at the ends of the wires you find joined and you'll end up with the original red, green, black, black set completely unhooked and 4 pairs of wires that go directly to speakers. I did some extra cutting at this point to make even-length pairs as they were all different lengths in the bundle so that the splice points weren't all in one place. I left the individual pairs different lengths, but do whatever works for you. You'll have plenty of wire to work with when you're done freeing them

I won't mention wire colors - they seemed to vary a lot from year to year and I know at least on mine, the colors were hard to discern anyway. The fronts on mine were pretty obvious; they were the pair that just disappeared left and right behind the dash - the rears disappeared back into the wire bundle under the console. A quick test of one of these pairs told me which one it was which made the other obvious. It was all straightforward butt splices from there on out and everything was good to go.