Q: Keyless Entry Problem on 2002 Ford Taurus

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My keyless entry remote will not allow me to disable my car alarm system. I believe it is an after market system, however, it does have the flashing "THEFT" light in the dashboard. I bought it used, so I'm not sure about that. When I go to turn the key to start the car, the alarm sounds and the car will not start. I also have a few electrical problems (speedometer, overhead light, radio, lever to the hood and trunk, tachometer and odometer, won't work). I need help immediately!! PLEASE!
(1) Answer
Go to a Aftermarket Stereo store maybe bestbuy and have them disable that alarm and possibly install a different system