in .OnInit StrCpy $INSTDIR "c:\MYDIR" has no effect?

[bdbaddog]

This works

code:
Function .onInit

    InitPluginsDir

    !insertmacro MULTIUSER_INIT

    StrCpy $INSTDIR "C:\BLAHBLAH"

FunctionEnd

This doesn't work

code:
Function .onInit

    StrCpy $INSTDIR "C:\BLAHBLAH"

    InitPluginsDir

    !insertmacro MULTIUSER_INIT

FunctionEnd

It looks like "!insermacro MULTIUSER_INIT" must overwrite $INSTDIR