Hi
I would just clean up all the battery connections first, then check that the earths on chassis and engine, also clean up the alternator terminals and see if this improves the situation, before spending any money
If your secs monitor is powered by the ECU feed this means the ECU itself is not getting NBV (normal battery voltage) This is not good. If this is the case you may have to replace the feed cables from the battery to the fuse box and ignition switch etc. See what happens after a clean up.

Mike.