The the unit was installed by a PO, it's possible they didn't do the best job. Try pulling out the unit and checking the connections on both the constant and switched power, additionally, if there is an aftermarket amp, check the switched power lead from the head unit to the amp for a bad connection. Also, check the wiring carefully from the head unit as I've seen some people just jam the unit in and wiring can be damaged on the metal sleeve (insert) that holds the unit.
Reason #654 why I hate PO mods...you never know the quality of the installation.
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