Finally had a look under the dash of my 183 to see what's needed for the upgrades (2 520C's, one in dash one on bow) Looking to network both, and share an antenna.
Found one great thing! The power cable for the X-135 & X125 that were supplied in dash & on bow, are that of a higher grade Lowrance (exact as the 520C power cord! ) I thought the X-135 & 520C power cords were different (guessing it's a 520C cord, that will fit a X-135, but a X-135 cord would not fit a 520C, so they supplied it with the higher grade one)

Now here's what I found:

NOTE: *ALL cables mentioned below have blue ends*
Coming from the main harness out of the side of the boat (under the console) is a cable, that is attached to a "Y" cable (I take it that these Y cables do the same job as a "T" connector). The 2nd end of that Y cable is not attached or terminated (just the blue end, hanging), and the 3rd end is attached to another Y cable. That Y cable's 2nd end is terminated, and the 3rd is attached to a blue ended power cable (guessing it's a power cable because it has an inline fuse on the + wire). That power cable feeds back into the main wiring harness, and feeds back into the side of the boat.

The power cable for both the bow and console only have the sonar unit's wires powered. The nmea 2000 & data are taped.

What do I have going on under there???