It's either a bad power or ground connection with the HU. Check all the connections are tight, clean, good solders etc. If so then you may have to do what Jesse said and run a seperate power straight to the battery. If you do that you'll want to run a ground of the same gauge.
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Ill say i go by His and Al's thoery of "Your getting sprayed" Doesnt matter what it is. Grandma in the 1982 LTD is gettin the bottle.