Remove the cover from your 2001 Chevy Silverado stereo. Remove the stereo retaining bolts. Remove the wiring harness and the antenna cable from the back of the stereo.
It has 4x6" speakers.
All the speakers in a 1991 Chevy Silverado should be 4x6 if they are stock.
Yes they are interchangeable.
Generally, a stock silverado will have four speakers; two in the dashboard and two behind the seat.
No, Chevy changed the wiring harness in 2003. and the way it is mounted, it can be made to work though.
With an adapter Yes.
6.5" round speakers.
Front Door Speakers are 6-1/2"
You don't need factory speakers. Just buy the same size speakers and install.