Easy way to probe signals on a multi-conductor cable?
do these wires have a termination? you have some other options one of your comments in your posts mentions piercing probes, depending on your cables this could be and option... used in cars often.

I don't know how it is connected, because it helps to decide what is best, I think you should look up back-probing and piercing and see if any of those is worth the risk versus making a termination for all 20 wires yourself.

this video has a simple explanation