Check the connections at the megafuse, below the outside fusebox; that's a common spot for problems.
Checking the voltage at the fusebox and tracing back while the blinker is making the voltage vary makes it easy to troubleshoot; when the voltage stops varying, the problem is between the last two points.
If it varies more than a volt at the battery itself, even with the engine off, your battery is wasted, or barely charged.
Charge the battery and check again; that's a sure sign of a cell that's slowly dying.
If charging the battery fixes the variation, the alternator isn't charging enough, and needs rebuilt; I've used "Victory Lap" kits from amazon with no issues. ~$20.
You may end up replacing the battery too, if the Alt has been undercharging for a long time.
Red '96 Cougar XR-7 240k mi. '02 4R70W, PST DS : '03 PI engine, 04 maf, 24lb injectors, 2.5" exhaust, '02 4r70w + Jmod, DirtyD0g TC + cooler + 3/8" lines, 255 walbro fp. Alpine system.
Black '96 Cougar XR-7 (Lazarus) 210k mi PI Intake, '02 4R70W, Jmod, PST DS, GrogTune, Konis, Mark LCA+Poly, racecougar Custom Engine Chain, and JL and racecougar Bracing.
Black '97 Tbird Limited Edition, '02 4R70W, 255 walbro, PST DS, PBR Brakes&SS lines, Toicko Blues & Springs, GrogTune.
I'm not Human enough to be human; but I'm trying to fit in, and I'm learning to fake it.
"Never said Hibbity..." - Brian Williams