2004 mercedes ml350, sub, amp

ummmm..... I Am An Idiot, did I read your posts incorrectly or did you just do a 180 on the LOC?  You stated that you should run the LOC at the Headunit so that you don't pinch speaker wires running all the way back to the amplifier....

"Reason being, with a converter connected at the back of the radio, there is very little chance to ground a speaker wire.  Running speaker wires to the amplifier, depending on where the amp is mounted, you have a much better chance of driving a screw through it, or pinching it under a seat bracket, pinching it under the door sill" 

.... which sounds odd, considering that you'll be running an RCA in the same manner, with the potential to get pinched, etc. etc.  Then you told him to hook up the LOC at the amp...

"Where do I hook up the speaker wires from the LOC and the remote wire from the amp? Do I connect them at the stereo or the amp." 

"At the amp."

Not trying to be Johnny Do-good, but it sounded confusing.  I usually install the LOC at the best location, whether at the head-unit, if I know that it's a full-range out, or at the amplifier with speaker wires running to whatever speakers I pair off of.  If the ML350 has the Bose system, then you'll have to run the LOC off of the speaker wires at the speakers themselves.  Or you can tap them before going into the door boot.  If the Bose system has a factory sub, then you'll need to tap the sub for the signal, as the front and rear speakers will be high-passed at the amplifier.  If there's no factory subwoofer, then use the rear speakers as your signal because the fronts will still be high-passed.  You can tap the rear speaker wires in the B-pillar, then run them back to the amplifier and connect your external LOC.  Do you have to use an external LOC?  It's all dependant on the installer.  I use adjustable LOC's because not all headunits are created equal.

Be careful when finding your speaker wires in the lower  B-Pillar as the speaker wires AND the Seatbelt Safety wires are both twisted pairs.  The Seatbelt Safety wires will instantly tighten the seatbelt to help in a collision similar to an airbag.  Pull the door panel and check wire colors to verify you have the correct wires in the Pillar.  I was installing a amp/sub in a ML320 a few years back, found two twisted pairs, tested both of them with my meter, which both read around 3 ohms, and when I connected the amp up, BAMMM!!!! the seatbelt retainer fired off and hello to a $490 repair.