You would be far better off buying even a cheap in-car amp, as the guitar amp you have will be mains powered (?) You could run a voltage inverter via the car battery and that will power it, but the amp will be a miss match with the speakers in the car in terms of Ohms and probably wattage. The guitar amp can be unsoldered and then re-wired to the car speakers to bypass the built in speaker. The amp would likely not be powerful enough to power the sub properly - let alone speakers on top of that! It is possible, but a completely pointless venture if i may say so.
Grab yourself a pukka in car amplifier, and you will be much less stressed!
Hope this may help!
Regards,
K