There are two adapters you can us to feed the rear signal to the rear/sub amp. If the HU has RCA's then there is an RCA input adapter that feeds the clean low level RCA signal to the rear amp. There is also a high level speaker input adapter that takes the amplified rear speaker output from the Sony and converts this back down to pre-amp level and feeds this "dirty" signal to the rear amp. The signal is dirty because it's already been amplified, then reduced down, only to be amplified again by the inferior rear amp and is prone to distortion, hissing and interference. Both will work but without knowing which one they've used it's hard to say why it's cutting out. It's more likely they've just bodged the remote on trigger connection probably with a piggyback splice and it's making an intermittant connection and turning the rear amp on/off randomly. Just pull the HU and post a pic of the adapter and wiring, it's an easy fix.
