CSR8645 3.2 bluetooth module audio output merge negative pins
The outputs are likely to be BTL (Bridge-Tied Load) meaning the negative outputs have a signal at the same DC level but opposite in AC polarity.  This adds 6dB to the max output level and eliminates the need for coupling caps.

If so, in order to use them single-ended, ignore RN/LN (do NOT ground them!) and capacitively couple RP and LP (which I assume are the names of the positive outputs) to your line inputs.  With luck, the amp CSR put on there will have fairly low common-mode noise.