If you want to replace those worn or rotted interior door panels and rear quarter interior panels then here's a cheap way to.....Git-R-Done. You will need a sheet of 1/8" thick fiberglass, like the kind used to underpin mobile homes and subframe houses, a hot glue gun, a pack of hot glue sticks, and the material you entend to use to cover the panel with (preferably marine grade vinyl). Tools that are required are a jigsaw with a fine cut blade, a drill with assorted bits, and a black Sharpie.
Remove your old panel and remove any pieces that will keep it from laying flat on the fiberglass board. Now line up any square edges with the straight edge of the FG board then trace a outline of the old panel onto the FG board. Carefully cut out the pattern and then test fit it to the door/quarter that it will be attached to. Keep in mind the trim piece at the top of the door that the factory panel slides under and allow enough excess to fit back under it. Make sure not to leave any sharp edges on the FG board because this will cause premature splitting of the vinyl.
Once you have the desired fit you will need to go back and mark any holes that need to be cut into the FG board like speakers, door handle, window crank, power switches, arm rest, ash tray, etc. BE SURE TO MAKE THE HOLES THAT THE DOOR CLIPS FASTEN THRU TO HOLD THE PANEL ON. YOU CAN USE THE PLASTIC TYPE LIKE USED ON NEWER MODEL VEHICLES. THESE CAN BE FOUND AT MOST PARTS STORES.
If you are adding something that was not orginally in the panel like speakers make sure the location you pick on the board will match the body of the vehicle and not be in the way of a lowered door glass or wheel well.
Now that you have your pattern finished double check to make sure it is clean of FG shavings or any type of trash that will show up on the finished product. Also, if the FG board has one side that is smoother than the other then it's best to use the smoothest side.
Take the material you plan to cover it with and stretch it out over the board. You will need to leave a 1 1/2 inch excess around the outside edge this will be used to glue the material to the board. Once you have the material trimmed flip the material up side down and place the board down on top. MAKE SURE YOU HAVE THE RIGHT SIDE OF THE BOARD FACING DOWN ON THE MATERIAL OTHERWISE YOU JUST MADE A SPARE.
Take the hot glue gun and make a bead across the top edge of the board then pull the material over and press it down. This may take a few minutes of rubbing to get the material to stick but once the glue cools it will stick. Now gently pull out any wrinkles in the material and place a bead of hot glue along the bottom edge and lay the material over it making sure you do not leave wrinkles in the material. You can gently move the material if needed just don't try to give it a hard pull. Once the bottom has cooled double check the covered side to make sure everything is looking smooth. Now do the sides the same way. On the rounded edges you will need to make relief cuts on the back side so that the vinyl will not crease on the top side. This can be done by cutting a straight line from the outside edge of the vinyl inward towards the FG board edge and stopping at least a 1/4 inch from the edge where it wraps over to the top side.
Once the material is in place go back and take a new razor blade and cut out the holes that you made in the FG board. Be sure to put a small amount of hot glue under the edge of the cuts to help hold them flat. THERE IS NO NEED TO PUT ADHESIVE SPRAY ON THE COVERED SIDE OF THE BOARD. THE GLUED EDGES WILL HOLD THE MATERIAL IN PLACE.
Now place the newly covered panel on stand back and admire a job well done!