Sorry to be so slow! I just opened up the mic head and removed the contents, unsoldering the old element. I cut a new piece of foam out of foam sheet I had around to fit inside the mic head and also cut a hole in it to fit the new element. It's not very neat looking, but it works. Then I just soldered the mic wires to the new element and stuffed it into the hole in the foam, making sure the open part of the mic cartridge faces toward the front of the mic (it sounds silly to point this out, but I have made this mistake before myself). The element I used is an electret which came with the V5 kit. It's polarized, so don't put the microphone case back together until you have verified that it works. My cartridge was not marked for polarity, and predictably I guessed wrong the first time and had to swap the leads.
I reinstalled the baffling that was in the old microphone - some fiberglass and a rubber sheet of some kind.
Depending on what's in your existinng microphone head it might not be necessary to install new foam - just take out the old cartridge and put in the new.
The D-104 head is very easy to disassemble - 4 screws with slots on front and back of the circular head. Taking the head off the stand first makes it easier to handle. Of course you will have to make sure the three wires involved (two to the mic element and on to PTT) end up at the right pins on the plug that goes into the radio.
Hope this helpful. If not, just ask.