F15 is for the key warning system, anti theft alarm system, courtesy lights, automatic antenna, trunk light, and power seat relays (not the seats themself, but just for the control of the power seat relays in the fuse block). I would guess that the problem is in the courtesy light system or antenna. This power goes to all 4 doors (lights in the bottom of each door), the trunk light and antenna motor, overhead lights, reading lights in the rear, etc. Try to think back if anything was done in any one of these areas just before this problem started.

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84 500 SEL (307,xxx miles)