Howard, the easiest thing (IMO....) to fix all this at once is..
GET A NEW UNIT.
A new unit will save you trouble of having to order O.E. stuff that will fit to get a nice system........
New units come with a built in amp (Most of em), that way it bridges with the current amp in the car (its like 50 watt amp built in) - and pumps almost 75 to 100 watts per speakers. Then, your car can handle any speaker in the front/back.
The stock O.E. unit clunked out when i got new rear speakers... Might have been installer or something. DAMN YOU BEST BUY.
-Phil
P.S. Hows the intake working for you?