Ok I will assume you have two power wires, One may be gray if I recall correctly the other red. the 3.5 amp is the gray lead (or some other color)
The RED lead is power. It likely gets fed by a fuse on the fuse block somewhere,
True story: I live in a motor home, the house radio worked fine but the dash kept cuting out.. they use the same speakers,,, Well I traced it to a bad connections somewhere near a high power (150 watt) Stereo amp down by the parking brake pedal on the left kick panel... Turns out there is a relay box there (I could hear it click when I turned the ignition key) it has two multi-pin MOLEX locking connectors.. One (house) was locked, one (Dash) was not.. I pressed, heard first a CLICK, and then Peter Paul and Mary playing (That's what I had in the Dash-cd at the time).
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times