Try the following diagnostics bit:

Grab a terminal program for the SE/30. ZTerm should do the job. Set it up for 300 bps, 8N1.

Connect the cables to your IIc. Make sure the cables are connected to the modem port and not the printer port. On the IIc type:

IN#2

6B

(Ensure that everything is upper case, to be safe. Stuff in <...> refers to a specific keystroke.)

Type something in the terminal program on the SE/30. Is it showing up on the IIc?