Originally Posted by PeteS160

You're fine to splice the wires. just keep them tightly grouped and away from interference.

Originally Posted by PeteS160

As for switching the signal...sure it can be but not with a switch it self. A switch is designed to open or close a circuit for electrical current to flow though. The serial data line is a transmission line not a power line, to prevent interference or skewed readings use an automotive 5 blade relay tol split the 2 DLC ports. signals. Singel from the car would come in on terminal 30 of the relay, DLC 1- terminal 2(Serial Data) would be on #87A of the relay and Dlc 2 - terminal 2(serial data) would be on 87. Now wire a switch up to the relay on terminals 85/86 and boom you have a switching data port. When the relay is off DLC 1 would have the active data line, when the relay was turned on DLC 2 would have the active data line.

If your wanting to switch a bunch of lines look on Amazon or Ebay for "Arduino Relay Module". It's relay boards designed to switch multiple relays with a single switch. You can get big one's....I picked up a couple of 16 relay boards for around 10 bucks each this past summer. So with 1 switch it controls 16 relays that are built into the module.

Originally Posted by PeteS160

If your smart about what you use you don't need to switch any data lines. If you have 2 devices plugged in at the same time all that's going to happen is neither device will work correctly until you unplug one of them.