Mapping in .vimrc doesn't work

[Wim R. Crols]

Hello,

I use an AZERTY keyboard, where the backtick is reached by pressing 
AltGr + µ.
To make using it easier using backtick I added a mapping in my .vimrc
   map µ `
But this is never registered when I open Vim. It does show up (in blue, 
not sure what the colors mean) when I type ":map" but it doesn't work.

Strangely enough, when I input the mapping again directly when working 
in Vim, it works perfectly. When I type ":map" then, it shows up twice, 
once blue and once black.

Any ideas how I can make it work in my .vimrc too?
I have this problem under Windows and Linux

Thanks a lot,
Wim