uhm... ok where to start...

depoendant on the model and radio fitted to your car you may have a combination of possibilities here...

most likely (as you have refered to dash speakers) is two speakers in the dash, two in the rear doors (possibly 2 in front doors as well - but liekly to be blank grills)

the likely installation is the two dash speakers being driven from th head unit, the two rear door speakers being driven from the amp (mine is mounted in the dash immediately above your knee on the right hand side of the car)

the radio opening isnt standard din,,,duh but you can get plastic face plates that adjust it easily and cheaply...

the speakers are probably 8 ohm (mine were) and therefore will not (probably) match your head unit which will (probably) want 4 ohm speakers...

the speakers can be easily replaced in the doors with 5 1/4 inch (i used panasonic...but just what i got at the right money)..

my advice (from fighting with my installation) remove existing head unit (graphic if fited) and amp & speakers and head for nearest bin...

cut connectors off the wirring looms and solder you own (make it myself) loom extension on the get to the new head unit..wire in...insert new speakers and truck on!

sounds bad...isnt really...and if you go and buy yourself a haynes manual it has the wiring diagram in it...