Had a bad clarion amp in my 02 SAN. One channel would fritz. Thought it was the head unit at first, replaced that, same problem, replaced amp. Not upset though because we got a better HU and an inexpensive alpine amp works fine for the stock system.
You could check voltage at the pre-amp outs before they meet the amp. If you get nothing across a channel then it's probably the HU. You can also check each channel coming out of the amp, that's how I found my issue, no voltage from a channel.
We now have a CMD4, which works well, but I never had it rained on.
Question on how to check the amp for those who know. Could you put an ipod into the pre amp in and keep the gain low and turn it up to see if the amp is working??