Try a trackpad cable, but I doubt this will fix the issue. Also try booting with the keyboard disconnected, enter the password using an external keyboard and see if the trackpad still works after boot.
Sounds like an issue with the PCH to be honest, had one like this before where the trackpad and keys would work in the boot up selection menu, but when in an OS it would stop working.
Why did you replace the trackpad by the way? Was it working fine before you needed to replace this?