I always look for the - or NEG stamped on the battery case and make sure this is connected to frame ground. Trailers are normally BLACK for + and WHITE for - Frame Grounds but since those are also automotive cabling the color code might be RED for + Positive and - BLACK for Ground.
You may even find a RED WIRE connection on top of a BLACK WIRE Connection where the two worlds of TRAILERS and AUTOMOIVE items cross paths haha...
I had to replace my JENSEN CAR RADIO and right out of the box came with RED and BLACK WIRES... Where that connected to 12VDC was BLACK and WHITE color connections... Makes old timers turn over in their graves haha...
Finding the PLUS + and NEG - symbols stamped on the battery case helps me keep it straight for my setups...
If you find them reversed there are two FUSES in the power panel marked REVERSE POLARITY and are usually off to them selves like the the 40A fuses shown here. They will most likely be blown...
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Roy Ken