When the volume goes down does it make a difference if it is on AM or FM?
How old is the battery?
How old is the radio, would you guess? Is it an 8-track radio?
With the trailer plugged in (110v), what is the voltage at the battery with nothing on? (no refer, no lites, nothing)
Are the wires to the radio accessible? If so what is the voltage at the radio? (with radio off)
Then turn on radio, and looking for voltage at batt and radio for both times the sound fades. ~~~~~ Do the routine - turn on, sound fades, measures voltage, turn on lites, sound fades, measure voltage.
1966 Mercury Park Lane 4 DR Breezeway 410 4V, C-6, 2.80 - Streamless.
1966 Lincoln 4 door Convertible 462 4V 1971 Ford LTD Convertible 429 4V Phoenix ~ Yeah it's hot however it's a dry heat!