all you need is!

a spare wiper arm
a vice
a hammer

take your spare wiper arm and put it in the vice.

lightly pinch the sides together so the hole has become narrow

then fit the wiper arm to the car and lightly TAP it down with a hammer so it fits itself to the spindle. if you have pinched it too much, grip the arm in a vice and beat it round aroudn and start again.

i used this technique on my own panda, as i said, use only if you have a spare arm as this can ruin the wiper arm. but it is an effect way to fix it with replace the whole unit.