If you attach a multimeter across CANH and CANL you should consistently measure 50 ohms as you wiggle various connectors.
If you have an oscilloscope, you can watch CANH and CANL versus GND. They should sit at around 2.5V when the bus is idle. If all's well you should see something that looks a bit like
this.
If you see inconsistent high/low levels or a super consistent message (50% high/low jabber) then something is wrong.
Terminators aside, it's very easy to crimp an RJ-12 terminated cable upside down and swap CANH and CANL. That'll instantly break a bus. If you hold the two ends of the cable up next to each other and look at the wire colors inside the plug, they should be in exactly the same positions on both sides. If they're not, re-crimp it correctly or throw it away.
CAN is normally implemented as a twisted-pair network. Most telephone cable is untwisted. If you're simply picking up too much differential noise across CANH and CANL, you could consider using CAT5 wiring. You should also move your CAN cabling away from wires going to/from motor controllers and batteries. Lack of twisting is why TI specified the max length to be so much shorter than the CAN specification for a 1 mbps bus.
If you post a scope trace of CANH and CANL, I can probably help you debug your circuit remotely.
-Sasha