Removing factory fitted head unit in a mira

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Hi

Now i really shouldn't be using this site as although i'm a british national, i'm currently living in Japan (hence why i have a mira) anyway after recently paying for my cars shaken (an expensive experience) i've decided to upgrade my incar audio.

Now the problem i have is I have been able to remove the facia around the head unit and all the offending screws facing me but i have a problem with the final screw holding the unit in place, its located under the radio and joins the radio bracket to the dividing strut between the upper and lower audio compartments (i am probably using completely the wrong terminology on that but i hope you get the jist) anyway this screw is a starnge shape, kinda like i need an alan key or something like that to remove it.

I'm throwing this out on here just incase other daihatsu models of a similar vinatge (i should point out its a 92 mira 660 12v L200) have the factory stereo fitted the same way and someone could pint me in the direction of the right sort of tool to remove it, otherwise a trip down to my local garage and my bad japanese and guestues is going to be required to get the radio out! All help most welcome, good site by the way!

Andy.