JLemond gave me the following advice on how to restore weatherstripping;
"ON YOUR REAR DOOR SEAL/ SEALS TAKE A LOOK AT THE ENDS AND THE CORNERS AND SE IF THEY ARE JUST FLATTENED , IF SO THEY CAN BE REPAIRED RIGHT ON THE TRUCK, IF THEY ARE TORN THEN YOU WILL NEED TO REPLACE THEM, HERE IS WHAT YOU DO TO FIX A FLATTENED ONE THAT ISN'T SEALING UP REAL GOOD , THIS WILL ALSO WORK ON THE SIDE DOORS AS WELL, GO TO LOWES OR HOME DEPOT OR ANY WHERE YOU CAN GET REGULAR WINDOW SCREEN LACING , THE RUBBER PIECE THAT IS PRESSED IN THE GROOVE OF WINDOW SCREENS TO HOLD THE SCREEN TO THE FRAME , THIS IS JUST A VERY SMALL RUBBER TUBE , NOW AT THE CORNERS OF THE DOOR SEAL, CUT A SMALL SLOT IN THE SEAL , THESE SEAL ARE HOLLOW THE FULL LENGTH ALL AROUND , TAKE THE PIECE OF RUBBER AND START ONE END INTO THE SLIT AND SPRAY IT WITH PD BLASTER AS YOU SLIDE IT IN THE SEAL AND TWIST IT AS YOU PUSH IT END ALL THE WAY TO THE CORNERS , THEN CUT IT OFF AND GO THE OTHER DIRECTION WHEN FINISHED WITH THAT ONE TAKE A TUBE OF SUPER GLUE AND GLUE THE SLIT BACK TOGETHER , BEST RUBBER CEMENT EVER MADE , IF YOU GO ALL THE WAY AROUND THE DOORS WITH THE SEAL LIKE THAT THE DOORS WILL CLOSE AND SEAL UP AS GOOD AS NEW . WON'T WORK ON THE WINDOWS SINCE THEY ARE SOLID ,BUT WORKS GREAT ON THE DOOR OR BODY SEAL. "