Originally Posted by EricLee31

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.
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It was all straightforward butt splices from there on out and everything was good to go.