Then I don't belive the cables have been messed up when installing your cruise controller.
Seems you can communicate fine with your ECU when the instrument cluster is turned off.

The solution I have currently in use is a cable from ECU's K-line to a separate
diagnostics connector plus a switch to separate Ecu from the original K-line bus.
Until someone provides a binary of his instruments cluster flash memory for analysis,
I don't expect a software solution will come up for this problem.