If it's not working with the harness, the only thing you can really do is a close inspection to make sure the wiring is hooked up correctly between the harness and the new head unit. If it is, you can try using a multimeter to see if the power wire coming from the harness actually has 12v on it or not. If not, check the fuses to make sure one didn't blow during the replacement. If fuse is good, perhaps the harness is not compatible? Double check to make sure you have the right harness.
If you are unable to debug it (or just don't want to) by double checking and using a multi-meter, about the only other thing you can do is what others have said... go back to stock equipment or run all new wiring.
If you are unable to debug it (or just don't want to) by double checking and using a multi-meter, about the only other thing you can do is what others have said... go back to stock equipment or run all new wiring.
