Well the amp I got with a sub-woofer I bought finally went out. I was wondering if I can wire up my aftermarket sub-woofer to the stock amp for the time being? I miss bass and I need it! lol. The woofer puts out 300 true RMS btw.

you could hack into where the subwoofer is connected to the amp and splice into it, wont be pretty but itll work

Has anyone ever done this? I assume it would not be recommended lol. Since the amp is underpowered, I assume so will the sub, will this damage anything in the process of usage?

damage what, the sub is already blown and if you plan on replacing the audio with aftermarket why do you care? if the factory am really is 300w RMS it isnt underpowered, im not sure but i doubt it is 300 rms, maybe peek, if it is 300RMS you can run just about any reasonable 10" or 12" sub

Since the amp is underpowered, I assume so will the sub, will this damage anything in the process of usage?
It'll be fine, provided you don't push it. Listen for distortion and if it sounds harsh, then back off on the bass. Nothing wrong with "underpowering" a sub, as long as you don't push the amp to it's limits where it's producing a lot of distortion.

damage what, the sub is already blown and if you plan on replacing the audio with aftermarket why do you care? if the factory am really is 300w RMS it isnt underpowered, im not sure but i doubt it is 300 rms, maybe peek, if it is 300RMS you can run just about any reasonable 10" or 12" sub

The sub isn't blown, the aftermarket amp is. The sub-woofer pushes 300 rms, and it's aftermarket, not factory.

sure, just get a diagram of the wiring that comes out of the factory amp and splice into it