Well they do sell "thin-walled" sockets, so it may be that you need a 16mm one, which is equivalent to 5/8". Also if bolt head is recessed and out of sight a small mirror might help you see what your trying to get a grip on.
I presume you have a manual to lead you thru' this job 'cause it looks complicated in my book.
Weird you have to add a spring with a new belt.
Good luck.
Edit: Simultaneous post seems to confirm your initial suspicions.
98 Mystique LS v6 atx 110,000km
97 Contour LX v6 atx 125,000km
stock CD on LS, cass on LX
spoiler, tinted windows on LS only
4 wheel discs on LX only
All other available options on both
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