The MK1/2 Mini with sliding door glass is considered the mother of all classic Minis. The glass slides inside a felt covered dual channel. This channel works great when in good shape but will deteriorate after years of exposure to the sliding motion and the rain and dirt of years of use. Replacing it should be an hour long task that will renew a 'classic' feature of your Mini!
These are the non flexible version that are as close to original as currently available.
33 inches FITTING INSTRUCTIONS 24A88 Mini front door window channel This window channel is now supplied without water drain holes. The drain holes can be identified as the larger 6.5mm holes in the chromium plated glazing strip. The smaller holes in the glazing strip are used for fastenings to affix channel and glazing strip to door frame. TO MAKE DRAIN HOLES IN THE CHANNEL: 1.Use chromium plated glazing strip as template. Mark drain holes on closed face of channel. 2.Using a 3mm drill bit, carefully drill pilot holes through closed face of channel as marked. 3.Using a 5mm drill bit, drill open from side of channel through pilot holes. (This de-burrs pilot holes allowing window glass to slide freely). New fixing holes may be drilled (using correct size for desired fixing) through open side of channel into glazing strip and appropriate area in door frame.