Did you try them with the inverter down/off?? I have some issues with my inverter...The heating pad does not work with it, sometimes use it for my birds..So, its most likely a resistor telling it no, not enough power..For example, when a portable radio sounds funky sound wise, the batteries are low right?? Say yes...So, sounds (no pun intended) that there may be a current issue with your electricity or whoever installed it, may have done a boo boo...Check all connections, make sure they are tight etc or have the guy come back and check it..If it worked before, why not now..Also, is it the same gauge wiring?
I had an amp issue back in the day in my car, changed over to a 6 gauge cable coming from the battery, fixed it,..was not enough power for the demand being restricted and it would blow the inline fuse...So, be sure the cable used is what you need too...What do you have?? any specs on it?? and are you sure you hooked it up right as well?? Maybe check on your end first?? If using a DVD and other things, try any speaker directly to rule out speaker issues..
Hope this helps some..
ANthony