Audio Systems: Distortion above a certain audio level, toyota landcruiser, pioneer headunit

Question
I have a 1984 toyota landcruiser that I've put a pioneer headunit in. originally it had a clarion, but I noticed at a certain level in the volume somewhere right before about halfway I would get a lot of distortion and cutting out. I replaced the headunit thinking that it was the clarion, but in the pioneer the same thing happens. I rewired the entire thing, direct connection to the battery, new accessory lead, tried different grounds. I recently put in a new aftermarket alternator. and I have brand new 6x9 speakers in the back that it happens to. I just cannot seem to locate what's causing the problem. Do you have any ideas what could be causing this?

Answer
Hi Grant,
Lot of head scratching going on here, If you have swapped the head unit from Clarion to Pioneer then the problem lies outside the head unit, lack of power is one cause but you say you have run new leads to both 12v live and 12v Acc and checked ground so that's out, New 6x9 speakers at the back rules those out too, which leaves... Speaker leads and front speakers, you don't say if you changed those, try disconnecting the fronts and try again with just the rears connected, worth a try. Is there an amp fitted anywhere, so many of these big cars have them inline with high level (speaker) inputs and that could be a problem, personally I don't like them and with modern head units running 4 x 50 watts out put there is little need for everyday running.
Anyway, those are my thoughts on the matter, poke about a bit more, maybe run some oxygen free speaker leads directly from head unit to each speaker. What ever you do, do one thing at a time and if it doesn't fix it go back to square one and try something else or you will end up not knowing where you are.
Have fun, Peter