The believe the solution the OP was trying to convey is to piggyback power from the lower display of the cluster, which remains on startup, to power the backlit LED, in sync with the odometer.
It could be done but it require precise soldering and physical alteration of your instrument cluster.
I don't really care but just stating that at a minimum, this would void your warranty and tampering with an odometer is a US federal crime of such.. I doubt but I'm not sure if this mod would be construed as such..
https://www.caranddriver.com/feature...to-the-future/