how to disable/set-text buttons in MUI_PAGE_INSTFILES?

[Brummelchen]

i have tried now several solutions and none worked.

;disable buttons
## Change Next button text
GetDlgItem $1 $HWNDPARENT 1
SendMessage $1 ${WM_SETTEXT} 0 "STR:----"
## Change Cancel button text
GetDlgItem $1 $HWNDPARENT 2
SendMessage $1 ${WM_SETTEXT} 0 "STR:Beenden"
## Change Back button text
GetDlgItem $1 $HWNDPARENT 3
SendMessage $1 ${WM_SETTEXT} 0 "STR:----"
ShowWindow $1 ${SW_HIDE}

this in a PRE function only works while the dialog is at work.
when all is done (incl my detailprints) all buttons shows like normal - woot

i want the cancel button as "Finish" and back/next disabled.

pls help.

PS even the header text isnt visible - only while in work.

here a shot how it is shown (fast capture) at work.
(the small pic inside ist after processing - not wanted)