Hey my Mum, yes my mum wants to put 2 subs in her vs statesman, the only problem is how do i hook it up and all and also she has got stock speakers and a stock cd player
so would i need to buy a new head unit and amp to power the sub ?

Well you definately gonna need a new amp and a bigone at that to power the two subs. Im sure you could hook it up to the factory cd player but i would get one with dual preouts it would give you amuch better sound.

Go to car stereo shops and look at head units that have dual or even triple pre-outs (make sure one pair is a designated sub output) then show the dude there the subs and buy an amp that will power them nicely, u could leave it at this but the big bass would drown the standard speakers out, get 2 pairs of 4inch splits to give some nice treble and mid range, should come out of the shop about $1200 poorer, about...

You would definately need an amp. you MIGHT need to get a head unit IF the current one doesn't have any rca outputs

once u get all the stuff then ask again how to do it and im sure u will get a response.
too slack to bother explaining it all right now. or someone will just show u

U can get line level converters that convert the speaker wires from ya hu to RCA outs... but i would only use the alpine one which is ~$80-100...even then it is a little dodgya dn only really required if ya hu is integrated with ya air-con etc etc...

Go for a hu with atl east front and rear/sub switchable RCA outs.... then if ya get 2 x 4ohm 10 or 12's and a 4ch amp u can bridge the 4ch's to 2ch's... one for each sub! Then if u want to do it cheap get a prefab box and aim it through the skiport... job done!

try not to spend anything under the 300-350 mark for a 2/4 channel amp