I went thru that exact scenario....except I went in by pulling the radio out...but either way...
You are looking for two pairs of 10 gauge wire, if I remember correctly. Maybe they were 8 or 12 gauge...I just can't remember now...but there will be two pairs of heavy red and black wires. Mine were not terminated, not covered in tape, not marked. Nothing. They were just laying loose in that cabinet behind the radio and the go-solar sticker.
Use a flashlight, they should be in there. You might need to kinda dig around to find them...but they are a much larger gauge than the typical stereo wiring in that cabinet.
One pair comes from the solar connector on the roof, and one pair comes up from the battery, thru a circuit breaker mounted near the converter in the cabinet under the dinette seat, ON MINE. Yours might be different, of course.
When you find them, use a meter or circuit tester to find which pair is hot, 12v, and the other should only show voltage if and when a solar panel is plugged in on the roof connector.
When I called Forest River to ask about this exact issue, they told me to read them the VIN, and when I did they told me that 'you are not the registered owner so we can't help you'...
Idiots.