is it a mk2 golf then??
me cousin had 2 of em...and wel..there were tweeters alredy mounted in one v em...but appeared as if they were mounted afterwards by the previous owners. although the door cards on the mk2 appear like there is no space to mount things behind near the top..theres enuf room to route cable through if you pull the panel away temporarily..make a hole n jus stick/screw a tweeter externally onto the door....may sound bodge but it looks ok. other way is to fabricate your own tweeter housing perhaps?
in the event of puttin it on the door..u'l have to run the wiring neatly through the door hinge area or the loom which connects any other electrics from the door (if applicable)