Sounds like possibly a loose connection, but the shop could be right that it's an internal issue. Check the connections at the back of the unit. If those seem solid, you might check the classified section for a replacement.
(Of course there's other possibilities too, though less likely. It could be shorted speaker wires somewhere, but you'd probably need to dig into the doors to find that.)
MODS: PW TMIC, Cobb catted DP, HKS cat-back, AVO filter, Bren e-tune; Konis/Epics, Advan RCII