I'm in the middle of this swap myself on the 99. The lil sport mirror are furniture now, you could salvage the electrical connector but I'm going to use heat shrinkable quick connectors. You must buy a set of 99 manual tow mirrors and a pair of 03-06 power/heat tow mirrors. I bought both pairs from Wyatt Dodge off ebay... good price and service. The only thing I can add is now that I have assembled the (my power mirrors are stamped 06) 06 mirrors onto the 99 bases is that there is a approx 1/2" space between the arm and base on the bottom. Not a huge deal until I went to assemble the 03-06 none powered units to re-sell... dont go together, base is too deep/wide (oh well). Maybe 06 and 03 differ a bit. Anyways, works OK, just can't recoup $$$ by reselling. Oh, another thing I haven't seen mentioned anywhere else. When you seperate the base from the arm, take note of the 3 'ball bearings' inside, they position the mirror in its 3 swing positions (normal and breakway forwards and back). The grease may hold them in place and you might miss them like I did, had to assemble one twice. Oo.
Hope this helps.