For what company? Model ?
A quad cab has four doors where an extended cab may have just two doors or two full doors and two smaller, reverse doors. A quad cab is basically a crew cab, though Dodge has a quad cab and a crew cab. The Dodge crew cab is a bit more spacious on the inside. If you have people in the back seat of a Dodge, the extended cab will have the least space for them, then the quad cab and the crew cab the most.
either 3 or 4 depending on the year
The Chevrolet Silverado comes in both 2 door and 4 door models. The regular model has 2 doors, the extended cab has rear access doors and the crew cab has 4 doors.
6 1/2 in the front doors and 4x6 in the back if ext cab
Ford Rangers and with any other major truck or pickup comes with the option of an extended cab. The extended cab offers more room for the occupants in the backseat.
The front speakers are 6.5" (round) and the back (in the extended cab doors) are the 4"x6" size.
The crew cab truck has a lot more room inside the vehicle for seating. It has four doors and can seat up to six. An extended cab just has two doors (sometimes a third suicide door) with a little more room behind the main seat. Sometimes there is an extra seat or two that can fold up.
The 4 door became available in 2001.
1996 was the club cab, 1998 was the quad cab with the rear doors.
1976
4x6 in dash 6.5 in doors and 4x10 in the back
With the Extended cab models they come stock with 6.5" speakers in the doors and 4x10" speakers in the rear. 4x10's are goin to be a little expensive due to the odd size.