The Bose signals are low level already. You can just tap in to the existing inputs to the Bose amp and feed another amp straight from them, no converter needed.
The Bose wiring has four channels and a single shield connection which is *not* body ground - its a floating ground for the audio channels. Connect your RCA centre pins to the wires for the rear speakers, and the shield to the audio ground in the Bose harness (I think its white, not 100% though).
If your amp ties its RCA grounds to body ground you may get some interference, in which case you'll need to use an isolation transformer. You can get those online.
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