If you plan to do any nice audio upgrade you MUST replace the factory source unit. Call up
Crutchfield, tell them your priorities and see if they can suggest some things to check out on their website.
Take a look at the Sony RSX-GS9. It's about the most expensive head unit on the market right now but that is about the most subdued look of anything I know of. Price tag really only sort of makes sense if you're a super SQ nerd and plan to spend hours dialing in reference level sound and will really get some mileage out of all the built in processing power. Also you may have a hard time finding any in stock anywhere.
Not much you can do behind seats in a vehicle like that, especially if you have long legs and can't scrooch the seats up a little. You must take precise measurements and plan carefully to make sure you have adequate mounting depth for whatever woofer you chose. It doesn't take too much to pressurize a cabin that small but have reasonable expectations and whatever you do DO NOT try to cram too much cone area in too small a space. Smaller cone sub(s) in the correct airspace will always outperform trying to shoehorn a sub into an improper sized box.
https://www.qlogicenclosures.com/collections/q-logic-enclosures/ford+f-150+1994
If I got the year wrong Q makes them for most year F150s. Those allow very easy plug and play for any 6.5" component set you like and the imaging from mounting components in the kickpanels is excellent. I have run those in several vehicles and have been extremely happy with them every time. Personally I really like the JBL/Infinity lineup, but there's plenty of component sets in the 150$ range that should perform nicely on 80-100W. Toss a set of components in the factory locations and a set in those kicks on a little 80X4 amp and it'll be as loud as you'd ever want.
Cheap power is easy to come by these days and lots of it comes in a very small package. I buy expensive amps so I'm not the best guy to ask about reasonably priced 4 or 5 channel.
