Nobody is answering, and I think it's because there are too many variable to give a concrete answer. But I'm willing to throw out some issues with short explanations of why it's a problem.
Does your model have the dual 4" subs in the back? If yes, then I expect the amp is using two wires to one sub, two to the other. I'll come back to this later.
Does your model have only the 6" sub in the back (mine does), then if you open side panel and check the connector you would see that while 4 wires go out of the amp back to the sub, only two are actually connected to the woofer.
Both the above implies a single channel driving a single sub, each having two connectors.
Returning to the dual 4" sub case. It is possible that the amplifier could be used in bridged mode [one channel (-) wire combined with the other channel (+) wire], but it's also possible that trying to do that would fry the amp. I've never read an account of someone who successfully bridged the OEM amp (doesn't mean it can't be done, I just haven't read about it).
Complicating the issue is the Ohm rating of your subs. I know my 6" Nakamichi sub is a 6-Ohm woofer. I have read the later model ML models use a 12-Ohm woofer. Bridging an amp usually means the Ohm rating of the woofer(s) it drives has to rise. So if the dual 4" woofers are each 4-Ohm rated, an bridged amp might require an 8_Ohm woofer or risk death by overheating from current flow.
Long winded way of saying that before giving an answer, I think most would need to know your factory sub quantity/type, and the Ohm rating of the sub you want to use.