If you're connecting to an existing CAN bus/backbone, it should already be properly terminated. Adding additional termination resistors will lower the impedance of the bus and it may or may not work. If the added stub is less than a meter or so, there should be no issue, if it is longer than a meter well then you could have noise issues. You really can't add additional termination to the bus to mitigate the noise. Between CAN hi and CAN lo, with all nodes unpowered, you should be able to take an ohmeter and measure 60 ohms if the bus is properly terminated with a 120 resisitor at each end. (If all nodes are not unpowered you'll get some odd readings, sometimes negative). Normally when I do can in vehicle, I use a laptop running on batteries and connect only to CAN hi and CAN lo, using simple twisted pair. The ground is usually not relevant because the communication is based on a differential pair. If you have two ground planes that have substantially different potentials then you have and issue and should really be optically isolated. Adding optical isolation actually creates a "gateway" of sorts and becomes a bus in itself. I use this technique when the stub is much longer than 1 meter. The isolator would be within one meter of the backbone thus keeping the stub within the one meter tolerance. Now that you have effectively connected two backbones through the isolator the new backbone will have its own termination so it can be as long as.... I don't remember how long a bus/backbone can be off the top of my head. Caution, don't over think this, the CAN backbone should be terminated at each end of the bus. Generally a stub is not terminated. So if you are connecting at the end of the bus, removing a terminator and extending the bus, this is not a stub connection and needs to be terminated. If you are connecting between the ends of the bus, this is a stub, and should not require addtional termination.
