if needed headunit (features) then amp , sub, front speakers, rear speakers
this is my basic answer to anyone who has a "simple car stereo question. ALPINE head unit or other 4 vole output preamp had unit. I have had them all, and I would rather buy 4 year old alpine than most.
cheap 4 channel amp on the bay. again i love alpine because their 20 year old amps just plain work. think 35x4. i also love are older sony. these can be bought for 100 bucks all day long and just work.
a single 12 inch sub in a small sealed box. if you can fit a 10 or an 8 in the trunk a 12 will fit as well and be twice as efficient, so channels 3,4 bridged will be plenty loud.
do not replace rear speakers unless they are actually torn and then replace them with cheap stuff. run these off of the head unit and keep them quiet.
buy the best front speakers you can afford at this time. you saved on the rears. run these off of ch 1,2 on the 4 channel amp.
you may able to keep the front speakers too, in my passat they sound so much better wit the mp i haven't gotten around to installing the focals.
got into cars via audio. i have had old school all active 1kw systems sony, kenwood, clarion,boston accoustics, rockford, audiocontrol, pioneer, alpine, ads,yamaha, soundstream, quart, focal, polk.....
today my car has a kenwood ipod only full color screen headunit.
simle alpine 35x4 running front stock passat spereates and a polk 12 in the trunk. rear speakers hooked up to the head unit.
i bought the amp used 12 years ago and it has been in 3 cars.