Many have done the Lock up switch mod and not reported cruise control issues.
I’d be looking into how you’ve hooked up the VSS as this will be required for the cruise to work, in these earlier Legacy’s the sensor is in the back of the instrument cluster, unless you have the turbo SS model where it will be on the gearbox if it’s electronic speedo like our Aussie delivered RS turbos are (the SS is the US version of the RS and early JDM GT, except US SS was only available in sedans I believe). Anyway, side track there.
Why not hook up an aftermarket cruise control system? Much easier than trying to retro fit a factory setup. I’m hoping we can get to the bottom of this one easily enough!
What year is the cruise control “ecu” module from? And did it come from an auto or a manual? If from a manual it’ll need a clutch switch bypass - probably just a wire across those pins to say the clutch is always engaged (pedal is at rest position).
Cheers
Bennie