Nearly 100% of all ground loop problems are installer related, if not personally then simply due to lack of experience. While it is true that bad RCA's could potentially be the root cause, they could be using CCA power and speaker wires instead of OFC or something as stupid as not soldering and shielding power crimps or a wire splice. The shop may simply not have the expertise to resolve the issue. Ground noise is a common question I receive PMs on and while it's not a quick fix as there are a dozen common variables, it should be childs play to any experienced installer with hands on the vehicle.
IF this is
Cartronix in Valparaiso, I would have some minor concerns as Brad Marvin and Brian Trinoskey are no longer employed there. Brad now works for Rockford Fosgate and I am unable to confirm Brian is still in the industry. Regardless, I would connect with the owner, Eric Carter, before consulting an attorney. While Eric owns the shop, I am unable to find any acolades involving installation on him or his new installer, "Mark".
I would also contact
Mobile Electronics Mag as this shop is (was?) one of the top audio retailers in the country (13-15), so they are aware. If an attorney consult is less than $50 as they are in Texas, I would recommend one after speaking with the owner of your installation shop. If of course, you are unable to receive an amicable solution. There is no scenario whereby paying a pro shop of this caliber, should a client leave with a non-working system, defined by the clarity and level of equipment your describing.