I agree, Wayne, that the MM shares many components with the GM, as it also shares with the CV/PI. Its all FMC, yes? But, I disagree that the MM is just an option package on a standard GM, so, let's leave this here.
My point is not to debate the MM heritage, but to point out that LM didn't design the MM to be a tow vehicle. There is no tow package option on the order sheet, and that may be something to you to consider.
www.mercuryvehicles.com.
This LM site has the specifications for all 2003 cars, and clearly, the tow capacity of an MM is intentionally minimal; "Class I, 1500lbs, total weight not exceed 6600lbs." That's telling, and I encourage deeper thought before towing anything heavier than a pair of jet skis.
If you wish to install a Class III and tow 4500 pounds, the MM will probably pull it off. But, when she breaks, better drop your hitch assembly before calling the tow truck, its presence will imply abuse over manfacturer's specs, and that may void your warranty.
A Class III hitch is risky, but the same risk I took when upgrading my MM for performance. Now, I'm banking on the fact that an LM dealer installed my 4:10 and stuff without a whisper of foul, but I'm still rolling the dice. I paid more that I should have in labor, but I believe I'm buying a bit of insurance by having the LM shop and certified techs do the job. It may be worth a few extra coins to buy a new hitch through a dealer and have them install it.
That's my point, gentleman, thanks for listening.