Car amps like this one have two sections.
1: The part with the multi-winding transformer toroid dougnut inductor
This DC/DC supply makes someting like +and- 30volt for the amps for more output power, because straight 12volt on the amps would only be 20watt RMS (4ohm load, bridged amps) per channel.
2: The actual amp(s).
Find two big caps with a voltage rating over 25volt.
That's the output voltage of the DC/DC converter.
Measure that voltage.
It will be there if :
The DC/DC converter is in working order.
And the amps are not shorted.
And there is 12volt input.
And the power switch wire is connected to 12volt.
The main power wire of a car booster is permanently connected to the battery.
A thin switch wire (yellow? or red?) turns the amp on or off.
This is normally connected to output of the power switch in the car radio, the motorized aerial output (yellow).
Turn the radio on, aerial goes up, and amp powers up.
Intermittend working could just be a dryjoint.