Troubleshooting an Alpine MRV-1505

Did you check the RCA shields on the sony for continuity to ground?

Was the sony being used with the alpine at one time?

If the alpine had an intermittently shorted transformer, that could cause it to damage head units.

With no signal to the RCA inputs, measure the voltage from the primary ground to the RCA shields. If it's 0v, try twisting, pushing, pulling on the transformers to see if the voltage ever goes higher than a fraction of a volt.