@Farhan
yar first off, it all depends on how much mony u r ready to spend. secondely i dont understand why people r telling u to get a 4-channak amp, when u r only going to use one channal. 4-channal means that the same power is divided into 4halfs.
hers what i think can be done, remove the current phata. call a carpenter and ask him to make a box, which should take up as much space as possible. place your woofer in there and please dont make any air ducts, as math needs to be done to make those holes and place a pipe in there. then get small boxes made for the speakers.
if your woofer is 7-800 watt then a 500watt solidaudio amp would be great and if the woofer is 1200-1500watt then go for a 1000watt solidaudio. and this amp should be single channal. yah pher 4-channal lay loo ager deck khasi ha.
get your battery changed if its old or if u r not changing it then clean the terminals.
make the bass in your deck zero, and adjust it on the amp for the woofer. this way, awas aunchi ho ge or phatay ge nahi.
dont know about karachi but in islamabad this amp would caust round 6-7k, new.
and rember u will never get that bass with a cng fitted car. still have any problems let me know