I have the M-unit, not installed yet, as long as my EHC is still working I will probably keep using it.
One guy on here installed one and it fried just like the stock EHC. I would recomend installing with a separate fuse to power the M-unit if you get one.
I don't see why the M-unit wouldn't work on your 2004 if wired up correctly.
I'm saving up to buy a 2nd M-unit for a spare, so if I ever have any problems with it I can just change it out quick and easy. If/when I get another one, then I plan to go ahead and install the first one.
Buying two of them is about the same cost as the wire plus or the PDM, without the wait.