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First off remove the fender panel. There are some screws at the rear of the left front wheel well, a bolt under the small weather strip on top and one bolt about mid way back on the bottom. Use a thin open end wrench on the latter.
Remove the neg and then the pos battery leads and the battery hold down bolt on the rear side of the battery.
Since your ECM sits over the battery you may need to move it; I'm not sure as my '88 doesn't have that feature.
Unless it is different than the eariler C4's, you should be able to remove the lower side body panel to get it out.
On my 86', I remove the 3 torx screws in the wheel well, loosen the rear bolt on the bottom (a wrench will slip in the gap), and I think there is one bolt on top on the rear of the panel. After that you just unbolt the battery cables, and loosen the hold down clamp and slide it out sideways.