1998 Isuzu Amigo Speedometer and odometer quit working

The speedometer started jumping erratically when driving and then finally quit completely. Check engine light now stays on, but vehicle seems to still run OK. Dealer swapped in a new speed sensor and said it didn't fix the problem and said I have a signal going to the ECM so problem is within the ECM or between the ECM and the cluster. I see there are many other posts related to various Isuzu speedometer failures. Should I buy a salvaged ECM and try replacing mine or try a replacement instrument cluster? Would a service manual tell me what wires/terminals to measure to isolate problem? Could the problem be related to a service bulletin I found for 1998 amigos/rodeos to an improperly crimped ground wire be the problem? (Remedy in bulletin is to replace the wiring harness).