How to reverse polarity on only one device?
2nd circuit is probably better, though I'd consider ditching RLY3 and use two diodes instead, assuming you can tolerate a diode drop in the power line to the accessories.







simulate this circuit &ndash; Schematic created using CircuitLab

ADDITION

However, I am a little concerned you don't appear to have shown any thermal sensing on the device itself to ensure the peltier does not cook.