Looking further at techknight's suggestion, I believe the current in the ground line should be low enough that it would work. So if you prefer to use the MAX4619 chip instead of the relay, the circuit would look something like this:
The cost is about the same either way, but the chip would be a little smaller than the relay. Both come in through-hole versions that are no more difficult to solder than the other connectors you've got already.
In your microcontroller software, at startup you'd look at the SELECT input to determine how to configure the CLK and DATA inputs. If SELECT is high, then expect CLK on line B and DATA on line C, or the opposite if SELECT is low.
This is the specific chip I looked at: http://www.digikey.com/product-detail/en/MAX4619CPE%2B/MAX4619CPE%2B-ND/1130171
The cost is about the same either way, but the chip would be a little smaller than the relay. Both come in through-hole versions that are no more difficult to solder than the other connectors you've got already.
In your microcontroller software, at startup you'd look at the SELECT input to determine how to configure the CLK and DATA inputs. If SELECT is high, then expect CLK on line B and DATA on line C, or the opposite if SELECT is low.
This is the specific chip I looked at: http://www.digikey.com/product-detail/en/MAX4619CPE%2B/MAX4619CPE%2B-ND/1130171