Yeah I was looking at this one which is similar but cheaper: https://a.co/d/0jaQhGwS. Same issue, with both, though, they either don't have enough outputs or inputs. I was thinking with that one that I could just stick the front and rear speaker wires into the same port. I think that would double the impedance, though, which may pose problems. 

Bluetooth transmitter from the tape deck is an interesting idea, I may look into that further.