Audio Systems: ic car sound problem, internal amps, crackling sound

Question
I have built a custom enclosure using 1 6x9 pioneer speaker instead of the useless rear speakers found in mr2s, i have wired the old speaker cable left + right to this speaker and the sound quality has improved, although when playing at higher volume i get a crackling sound which is most annoying as it doesnt happen every time, My stereo is rated at 50w per channel and the 6x9 at 150w peak.At first i thought it was a loose connection or perhaps i am sending too much power to the speaker. Any advice appreciated.

Answer
Two reasons (at least).

1) The deck does not drive a 2 ohm load (4 ohm speaker wired to each channel makes the deck 'see' a 2 ohm per channel load.
Not good, especially at high volumes.

2) You should use one channels negative and one channels positive to get 'summed mono' (not both positives).
Not all decks allow this, even when done correctly.

Best advice is to get a small external amp to drive the 6x9 off of one pair of the decks RCA preouts. More importantly, you will not be in danger of damaging this decks' internal amps this way. 50 watts mono should suffice, but it is easier to blow a speaker from too little power (being cranked up too hard cause its small) than it is with a large amp.