yea Sounds like the RCA outputs amp chip got hot and went into thermal runaway (basicly got to hot to work properly), this is NOT a PREMINENT condition, although could be indicative of a poor op-amp design
if its back to normal now, I wouldn't worry bout it, but a windscreen shade is probably a good idea
while car audio gear is made for 'harsh enviroments', it's not always made for an 'Aussie Harsh Enviroment'
Cheers
Bundy