What is being run on the high speed can network and what is being run on the low speed network?
All can wires are a twisted pair, usually a white wire and a white with a blue tracer wire, although that can vary.
High speed can is usually for ABS, SRS, Side object detection and some engine functions. Low speed can is usually back up camera and other mundane functions like interior lights.
There really is no "tapping" into these wires as you would throw off the resistance value which will render the network into an error and set codes. You can repair these wires, but you can not add on to or tap into these without causing problems.