Another update...
My eBay OEM mic turned up in the post today.
I had it checked, and it turned out its a passive Mic also.
The picture of it is in Page 1 of this thread, but you'll see it has a VERY short lead!
I got some high quality Mic cable today and did a little cutting and soldering - and then added a gold 3.5mm jack on the end.
I went through the whole procedure of removing the light / sun visor light / 'jesus handles' and fully fitted the whole thing (although I left the other one on the A-pillar for the time being).
Then I drove away to some dual carrigeway and called my fiancee. It rang a few times, then cut off. I rang again... the same happened. I tried again - where I got 'Look... I'm not joking - I CAN'T HEAR YOU. Honestly - I can't hear anything'.
Looks like my soldering has gone bad somewhere, or the Mic is just NOT compatible! Hopefully I have just grounded the wrong wire or something.
Its dark now... so I'll have a look again tomorrow.