The wiring on the mustangs is minimal at best, you need to run wires directly to the battery for your sound system, then for your ignition sense, 10v is too low with the blinkers on, I would suspect either some corroded connections or a bad ignition switch, this reading you gave must have been with the engine not running, if it was running you should have had at least 13.2v any lower than that and the regulator needs service or replaced. There is a link at the top of the forums on vacuum and electrical diagrams, it will give the colors of the wires and what they connect to, use your voltmeter, and check out some of the wiring, don't forget the grounds, like on the rear of the cyl head on the pass side, adding an extra ground from the block to the body might not be a bad idea for your stereo. And if you cannot solve the problem, you could add a relay to you ignition sense lead that turns on with the ignition sense lead, but one of the contacts gets power directly from the battery, and don't forget fuses. Good Luck.