I just upgraded my Transit with a Kenwood head unit. Replaced the side door speakers with JBLs + tweeters on a-frame. Added 6x9 JBL coaxial speakers on headliner. These 4 speakers are powered by a Rockford Fosgate 600watt 4-channel amp. 2 JBL 12" subs powered by an Alpine 500 watt mono amp.
I will be adding 4 more speakers to the house and all of these will just be hooked up to the speaker outputs on the head unit. This is so I can listen to things while I'm doing things in the back of the van with just the head unit turned on.
I went into all of this w/o knowing anything about car install and took my time doing it and I'd say it took about a month, but I was also trying to get all of my electrical wiring done to the dashboard area all at the same time. There was also a lot of the time I was waiting for parts and tools to come in. I winged it all w/o making any kind of wiring and layout plans so I had to pause a lot to think about things as I was working. Adding time was that I decided not to get an aftermarket harness, but directly soldered each wire to the van dash console. I guess you can say I took things 1 wire at a time.
Off the bat, the problem you will face w/o buying a new head unit is that most likely the factory head unit will only have wires that go out the 2 speakers. So the only upgrade you can do is replace the factory speakers. You need a head unit that has a separate output for the sub.
I will take that back and say that technically speaking, you could just wire a sub to one of your door speakers with the option of throwing crossover in between (so your sub doesn't sound too crappy and doesn't get damaged), but the factory head unit probably was not designed to handle that kind of load.
Better, you could hook up the sub(s) to the rear speakers outputs on your factory unit (assuming rear speaker outs exists on your factory unit and you are currently not powering any rear speakers). You could theoretically wire sub(s) to one or both of those rear speaker outputs. But you'll probably want a crossover in between so you're not sending high frequencies to the sub. Mostly likely you are under-powering the sub but should be somewhat ok as long as you are not too crazy with the volume knob. Some of the deep deep frequencies also may not be present because I don't know if your factory unit speaker outputs are filtering them out. I think it's kind of a hack job, but could work.
Most of the aftermarket head units these days I think have 4 speaker outs (for stock front and rear speakers). 4 audio outs (to power speakers with amps). 2 audio sub outs (to power sub(s) with amp(s)). The problem is replacing the head unit adds a whole new level of install complexity and adding an amp adds some complexity as well.
@qclabrat I don't think sound tech has changed much, but stuff like power management, navigation, backup camera, steering wheel control, etc.. has or has been added. On my car the 12v switched power (i.e. power when car is started) was non-existent because it is handled by some kind of a bus system. So to start, I had my head unit hooked up to one of the cigarette sockets which has a 20 minute timer on it. Now, I have everything powered by my house battery instead of the car battery and a separate switch system under my cup holders to turn all my sound things on and off.
@cdrmtbiker @johnbryanpeters You're free to come over to discuss or work on van things with me, but I live pretty far away, at least from Allaire, in Sparta.