That amplifier is 2channel. You need one channel per speaker (assuming you want stereo). Where exactly are the tweeters in the dash located? Sides, center? Depending on their locations, they may be able to be wired together with the door tweeters (if the tweeters are the same). Sounds like you need at least a 6 channel amp, or another amp that's 4 channel and use both. UNLESS: the stock system has a passive crossover network somewhere. Then one channel per side (tweeter and mid), so that's 2 channels, then another 2 channels for the subwoofer.