Assuming your FF is wired to the starting battery.... It sounds like a voltage drop issue with the motor loading down the 12V when it starts but several things can cause it. Start looking at every 12V connection related to the FF, including grounds and make sure they are all clean with tight connections. If you are running through one of those $8 resettable or self-resetting circuit breakers get rid of it and use an automotive type or a glass fuse. Have the battery load tested. It could be getting weak with enough power to crank the motor but dropping voltage in the process. The last possibility that I can think of would be the head unit itself, but I would not condemn it until every thing else has been checked.
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