Line output converter ground noise

Hi

I'm adding a couple amps to my 2005 Chevy Blazer.  I'm using a couple line output converters to drive the amps.  Everything works fine during the day.  At night when the lights come on (dash lights, and dim's the raido's display), i get a hum through the system.  Sometimes this is worse than others. 

I have the ground wire from the LOC's grounded to the same ground line as is going to the stock radio.

Any thoughts on this?